E.g. when an elder lies about taking their meds as prescribed, checking blood sugar, bathing, being active? I realize no medical person can read minds, but sometimes as caregivers, when the doc won't see what we see, it's enough to make you give up all hope.
If it is difficult to contradict your loved one in a doctor's office, it may be a good idea to send in a note ahead of time, expressing your concerns.
Certain times in the past when we've taken my Dad to doctors appointments he has seemed much better than "normal". In these instances I think that just getting out of the house and knowing he's going to a doctor's appointment makes him a little more "with it"! Normally we don't give him his meds beforehand, which might have a little to do with it also -- if he took his meds he'd be sleepy, etc. He will agree with the doctor, says he doing "okay", etc. Believe me, both of his doctors (GP and neuro) know that he's not "okay". That's why my Mom and I are in the appointments with him -- so that we can let them know exactly what is going on with him (he's in mid to later stages of Alzheimer's).
Many elder individuals don't want their doctors (or other folks) to know how bad they're really doing and won't admit to not taking care of themselves. Many just won't take care of themselves unless someone is there to help them, because they forget to take meds, etc.
One suggestion I would have would be to keep a daily journal of what the person is and is not doing, what problems they have, how they act out, etc. and take it to the next appointment. This will give the doctor a heads up. Also, most doctors will be able to tell what's going on with their patients through labwork, physical exams, etc. If you're concerned that the doctor is not aware -- don't be afraid to ask! They sometimes know more than we do and more than they let on!