My sister is 84 years old and has dementia. What kind of evaluation can I get to confirm she can not take care of her self?

One option may be if you are with her when she is delusional or having hallucinations, call 911 so that they take her to the ER. At the hospital make sure you speak to their on-staff social worker and make sure the discharge staff know that she is an "unsafe discharge" (use those exact words). The hospital will want to discharge her so may really pressure you a lot to come get her but do not. They may even promise to provide "help" but do not believe this either. Most likely from the hospital she will go into a facility.

Are you POA?
Do you or can you go with her to the doctor's office on her next appointment.
Make sure you are listed on any form giving permission for the medical team to talk to you. This is a HIPAA regulation, no one can discuss her condition if she has not given permission. Right now you can discuss concerns with the doctors office, send a letter to them or drop off a note. But until she has given consent do not expect a reply.
Depending on the advancement of the dementia it is possible that a lawyer will not allow her to sign any legal papers. They can talk to her and if they do not think she is cognizant you will not be able to become her POA. You or another family member can become her Guardian. If no one wants that role then the Court will appoint one.
But bottom line, right now there is nothing you can do.

If she already has a diagnosis of dementia from the doctor you could contact APS and report her as a vulnerable senior that can not care for herself. Call your States Elder Abuse hotline that will also get things moving. (self abuse, self neglect is reportable)

If she's not cooperative, you won't be able to do much (even if you are her DPoA) except report her as a vulnerable adult to APS (Adult Protective Services) for her county. They will move to acquire guardianship and then will take care of her needs and place her somewhere protective (a facility). If this happens then the assigned guardian will not allow transparency of her affairs anymore (so if you were taking care of her mail or accounts or her home you will lose access to all of it). Guardians are not hostile, they just take seriously that the person who is their charge has their privacy and affairs protected. You will still be welcomed to carry on your relationship with your sister in all other ways.

Hopefully you have all your legal ducks in a row and have a plan in case you go down the same path as she.