You won’t know if your parent is slipping if you don’t see them. As much as you think you could, you cannot really tell over the phone if they don't want you to know. Even if your visit consists of a leisurely conversation where they are not taxed or doing anything but speaking to you, you won't know.
You need to see how they do things or are doing with things. What does their refrigerator look like, is there expired food? Is there food? Are there too many of one thing (like multiple bottles of shampoo or are they going through things that should last a while quickly)? What does their medicine look like? Too many pills when there should be a refill? Not enough pills before it's time to refill? Check their closets, their drawers, are things a mess? (if they were always kinda messy this could be a challenge but check out the level of mess then)
Are they sleeping more or less? Are they withdrawing from social activity? Do they seem distant or defensive? Are they telling you things that just don’t add up? Are they falling for phone scams? Are their bills paid? What does their checkbook look like? Are checks bouncing? Are they made out to the wrong company/people? Are they in the wrong amount?
You have to be part detective here. But if you don’t want to see things or believe the things you do see, that is going to be an impediment to you. You have to be able to put all that aside. Put your feelings about your parents' possible decline somewhere else because YOU have to be the one to act.
As written about on AgingCare an estimated 60 percent of patients with mild cognitive impairment (MCI) and 81 percent of patients with Alzheimer’s disease exhibit some form of anosognosia (click for pronounciation). The word anosognosia is composed of three Greek roots, which combine to mean “without knowledge of disease.” They aren’t just trying to ignore it, they honestly have no idea they are having any issues.
And that means the responsibility falls on you. Speak with them about it in a kind, gentle way. Maybe ask them if they have noticed anything, but realize they may not think anything is wrong, so you may want to make an appointment with their doctor. Aging life care managers are also a great resource to evaluate their living situation and provide options for you going forward.