How do we find a consultant to assess my parents' needs and determine if in-home care or assisted living would be better?

Call your parents' Area Agency on Aging and ask how to go about getting a needs assessment for your parents. Or contact their county's Human Services department and ask for an appointment.

The assessor will come to their home. Be present during the assessment. Sometimes Elderly people exaggerate their capabilities, and they often believe what they say. It is good if someone who knows the situation can say, "Yes mom, you do cook your meals. But sometimes you forget to turn the stove off, or the oven on, or you don't notice that the leftovers in the fridge have mold on them. But you've been cooking for decades and you still know how! You just have a little forgetfulness sometimes while you cook."

Office of the Aging will come to your house for free and assess the situation. They are national so you should have no problems.

Social workers are usually pretty good at this. Place to start.

Your Area Agency on Aging is an excellent resource. Just had an assessment done for a friend but it was a six month wait. They will also set up in home assistance for you through their contracted agencies. Often in home help is free but is based on income so it may be a shared cost.

An Aging Life Care Manager (also called geriatric care manager) does exactly what you are looking for. You can find one near you by going to aginglifecare.org and clicking on the "Find an aging life care expert" button.

Usually these professionals work in small companies. Since they offer this as a paid service, chances are you won't have to wait very long. Their assessments and plans are comprehensive - this is really helpful to make you aware of things that you aren't aware that you don't know about. You can work with them over time to help guide you, as your parents' needs change.

Unfortunately there aren't Aging Life Care Professionals everywhere. Your area agency on aging will know of people (usually social workers or nurses) who do this kind of work. Your county health and human services may have social workers for this purpose. In my area, Jewish Family Services also offers care management. (You don't have to be Jewish to use their services.)

Good luck - it is so good of you to ask and take steps before a crisis happens.

Ask the social worker at your local hospital for a referral. We had an assessment done by the occupational therapist/physical therapist and a social worker. If you have a relevant diagnosis for one of your parents (physical or mental) , you might find the assessment is covered by Medicare if you get the doctor to write a prescription for it.
We also had an in home agency come in. I wasn't happy with them as I felt they were pushing their paid services without discussing other options for care.  Just be aware, they are running a business.  I don't think they were being unkind, but they were offering the solutions they had to offer- not necessarily the least expensive ones.  

I've just gone through this with ADRC and our local county Social Worker. My MIL exaggerates what SHE can do and is sure she needs no help. The SW met with her privately and then met with me after wards.
An in home agency would also have come out to sit down and discuss options too. I found everyone very helpful.
The hardest part is getting on the right track for asking for assistance.

My Mom's doctor has a case manager on staff who discussed my observations with me over the phone. She had a few suggestions that might resolve a couple of issues but then she suggested someone in our area who will come to the house and talk to my Mom as an objective third party. I've heard good things about this person but also know that he gets paid if someone is placed in assisted living so, I'm just keeping that in mind when we meet. It might be helpful to see if your parent's doctor also has a case manager who can give you some assistance and/or offer suggestions.

Ending up on list after contacting agencies? Hopefully you have already discussed this with their primary care provider - perhaps they can make a referral or get appointment with hospital social worker. If insurance is amiable, I personally would think the less changes at one time the better - so while waiting perhaps can get home-health care ... then while they are actually providing services they get a better assessment of your parent's needs. YOUR continued involvement essential of course to evaluate the care they are receiving and to better know if you feel comfortable with their assessment.

Both times I have had to provide support during an ‘assessment’, I have found that MIL and aunt treated the assessment as an exam that they didn’t want to fail. Their answers weren’t realistic, and the ‘assessor’ was very keen on hearing from them, not from me. As in comments on other posts, sometimes professionals go over the top on patient privacy and patient rights. Make sure that your assessor will be hearing from someone with current detailed experience of your parents’ functioning. If you and your sisters already disagree, try to make sure that you all agree on who is briefing the assessor and what detail is being given. Talking it all through in advance between you could be a big help. Good luck!