First, I would never confront an elderly person or someone with an obvious disability like Parkinson's and certainly not someone with a combination of the two. You're missing our point here. It is usually fairly obvious when someone is using a disabled space inappropriately, at least here under our blue badge rules it is. Perhaps it's different in the US if you don't need to put the hangtag in place until you actually leave your car to go off shopping, I don't know. A big section of offenders is the 'just popping to the cashpoint, nobody will mind' crowd, for example. They are usually very obvious. (disabled spaces in UK supermarket car parks are usually closer to the cashpoints than any of the others)
Second, I guess I'm not understanding why your father in law in the hypothetical situation above doesn't just keep his placard in the car. (I took mine out yesterday because I needed to renew it online so needed the badge number but otherwise it lives in the car so I *don't* forget it. I must remember to put it back in there today before I go out or I wouldn't be entitled to use the spaces and that's just my hard luck if I forget.)
Third, what are we hoping to accomplish? Awareness, for one thing. It's not all about 'getting people into trouble'. Parking in these spots when you are not entitled to do so is.not.okay. And some people genuinely don't realise that and think it *is* OK for short periods, like the 'popping to the cashpoint' example above. Nobody's going to ticket them in the two minutes it takes to do that. That doesn't mean they should get away without having this pointed out to them.
Pardon my non-quotey skills.
As it happens, you can't always tell by looking at my FIL that he has Parkinson's. Some days are better than others and a lot of his pain is a side effect of the medication he takes to control it. Now, as much as it would pain him to hear it, you can tell by looking at him that he is elderly. What surprises me though is that in this thread and others like it in the past on e-hell, invisible disabilities are emphasized as a reason not to confront individuals over their handicapped parking use. So to understand, the position of some people is that it is okay to confront people who don't "look disabled" provided they don't have the tag in their vehicle?
Also, you say it's obvious when someone is using the space inappropriately. Is that because of them not displaying the placard? Because again, if they have a placard, but have forgotten it, what do you expect them to do? Technically yes, they are inappropriately using the spot because the tag is not there, but the disability/pain remains. What's the point of confronting them? You said the objective was awareness, but I can't imagine these folks are not aware that they need the badge. They know. They forgot. Probably don't need a stranger chiming in to tell them something they already know.
And it doesn't matter why someone forgot their badge or why they don't have it in their car. The fact is they don't have it at that point in time. Once they have parked, once they realize they don't have it and once a stranger notices it is missing there is nothing to be done. We're talking about people being confronted when their badge is missing/not visible. We've already crossed the bridge of what someone should have done. If people always had it in their car or never forgot it this would not be an issue, but life happens and there you go.
Or is it the idea that people you visually assess as not looking disabled and not having the placard displayed so you can see it are probably not actually disabled and taking the spot from someone who does need it? Again, I don't think the majority of these individuals are unaware of the purpose of handicapped parking. They probably know, but are parking there anyway. Just like smokers know smoking is unhealthy and don't need strangers making them aware of the fact.
I respect reserved parking and understand the frustration, but there are other outlets available for addressing the issue that don't involve a confrontation based upon a missing placard and how disabled or not someone looks to you.