How do you determine when your loved can no longer feed him/herself?

I might suggest you consider:
Does she take any medication before the meal that could make her this way?
Is she exhausted by that time of day? Would a nap help?
Can she eat several small meals during the day instead of 2-3 bigger ones, so if she can't eat at one sitting it's not such a big deal.
Could she eat earlier?
Does she have dementia? If so, this is probably her worst time of day.
Would finger foods work better for her?
Would she drink meal supplements to help with nutrition?

You could just gently ask her, "Can I help you eat?"
I bet she will consent if she is hungry. She may not and that's okay.

She may not want to ask but want the help. Older people can be proud about those things. We have to step in and help to pick up the slack and do it without making them feel bad...not adding insult to injury, that is.

Maybe she can't see the food or is having coordination problems. This happened to my Mother. Of course, they are not going to say "FEED ME." That would be demoralizing. Just do it without talking about it or making a big deal of it. Sit to her right and have pleasant dinner time conversation while you feed her in an unhurried manner. It will change your evening:) xo

My husband is 87 and in late stages of dementia. He sometimes can eat on his own if it is finger food and cut small enough for him to handle. Sometimes he lays it down and drifts off and I have to remind him to pick up the food and eat. He used to be able to use the fork or spoon but no longer. I have recently started sitting in front of him and putting on his apron and feeding him. If he is full or doesn't want, he will tell me. Sometimes if I wait a few minutes and come back to him he will continue eating until he is finished. If I ask him if he wants something, he usually tells me no but this is because he doesn't understand what I am asking him. I find giving him few choices is best and just feeding him works well. I don't think he is losing his dignity by my feeding him. My goodness, I help with everything else and he knows I care about him. I think there comes a time when you need to step in and don't ask. As long as they can handle finger foods, let them try.

This may not have anything to do with you mil's issue with not eating in the evening, but this is great information that I just got from the nurse at the memory care facility that I just moved my mom to.
I was told by the nurse that dementia patients see as if they are looking through binoculars...try this....put your fingers around your eyes, as if you were looking through binoculars...notice how you can only see what's in front of you, nothing to the sides, nothing way up or way down, you need to bend your head way down in order to see if there is something on the table, say for instance a dinner plate.
I found this interesting because if I were to set my mom down for a meal, she wouldn't even know to eat, in order for my mom to get started eating, her fork needs to be put in her hand and then guided in to her bowl, so she knows it's there.
Just a tid bit of information that may help you or someone else out at some point in time!

Hi tinker13 - thank you for your response; all good questions. At this point we've actually gone four nights in a row like this and I'm beginning to wonder if she's sundowning. She does have dementia but it's not AZ....I don't know if sundowning can occur in other forms of dementia. (We believe she has LBD) I've commented to her every evening that she seems more tired than usual and asked if she napped at all...her first response is "I'm always tired", then she says she did nap for a "little while". Honestly I'm not 100% sure she did (we don't live with her but check on her every 2-3 hrs during the day) because she always comes up with the same time amount - "about an hour and a half".

She does more or less "graze" throughout the day. We don't push "the three solid meals" routine any more because she can't eat too much at one sitting. We do try to make sure she's getting something hot and nutritional at supper every night, which is always around 5:00.

She was drinking Carnation instant breakfast every day for a while as a supplement but got sick of it - the only flavor she liked was the strawberry. I tried going the finger food route...she asked me to cut it up. (It was some chicken that I had grilled up in strips...even gave her some dipping sauce with it) When I explained to her that it was made that way so she could just pick it up with her fingers she said "Well I'm not used to eating food with my fingers"...as if I'd offended her sensibilities somehow:-(

My concern right now is this "fog" she lapses into at suppertime seems to be turning into a regular thing. She's had the off day every now and then before, but this recent turn "feels" different. She appears to be her usual self earlier in the day, and manages her other meals and snacks just fine. The last few nights she's even seemed ok when we first get there, but as soon as I set her dinner in front of her she zones out. Last night I gave her a plate of watermelon cut up on a paper plate for dessert, and a fork and spoon to eat it with. It took almost twenty minutes for her to eat about the same number of melon chunks, then she sat hunched over the plate with the spoon in one hand and the fork in the other, staring at the puddle of juice and appearing to be trying to pick "something" up by pushing it with the fork onto the spoon. She was concentrating very hard on it! But at the same time she almost seemed to be drifting off somewhere else.

No meds just before the meal...last onemshe takes before supper is a Sinemet at 3.00. Her meds haven't changed in a long time so I doubt they factor into this.

She's still managing to feed herself, even through it takes a LONG time some nights and appears to be a struggle for her at times. I just wonder if there's a point when you have to say the struggle has become too much.

I think you need to spend the entire day with her to see what is going on and get a better grasp on what her entire day consists of. Could she be taking something you are not aware of? Other than that if I saw my mother was having this difficulty I would ask her doctor what he thought and in the meantime I would ask her, if I could help her, as she seems so tired. You might check her pulse for a rapid heartbeat, which will make them tired and needs to be handled with meds.

Another thing to try is to make sure the color of the plate is a very different color than the food. We used to use blue plates because we didn't eat any blue foods!
If her dementia is progressing, she probably is tired all of the time, Imagine how hard she has to work to do normal tasks because her mind doesn't work as well as it did.
You might also start thinking about the future. What if she continues to not eat? You might consider a dementia care unit. Sometimes people improve when they have help with daily tasks and activities to stimulate their mind. You also might want to think about decisions you might be faced with. What do you do if she starts losing weight because she isn't eating? Is a feeding tube the right thing to do? (That decision is different for every person/family) What does she want? How much medical intervention (for anything that come up) does she want?
Living alone may be too much for her to handle at this point in her disease. I wish you well as you think about her future. She is very lucky to have you in her life. The Alzheimer's Association of the Family Caregiver Specialist at your local Area Agency on Aging should be able to help you talk sort out priorities and help you find a way to make decisions.