Yeah, that's kind of a tough call. Like it was mentioned before, sometimes the form may seem incorrect to you, but for them it's just fine. Maybe they have an injury, limit of range of motion, whatever. If someone asks me about something, I'll gladly answer their question, but at my work it's a little tricky.
I work at a fitness facility for the university here in Chicago at UIC (stop by if you're in town; it's really pretty!), and I know a good deal of the patrons that come in. They see me working out there, and often mistake me for a personal trainer because of my working knowledge and able to explain without the technical jargon how things are done. I have to be cautious when I give advice at work because if they injure themselves after something I told him or her how or what to do, it would be me and our facility assuming responsibility, and from a legal point of view, this is not good. I have to refer them to a personal trainer for more info, but I always follow up with the points of common sense:
What may work for me, might not work for you. Every body is different and responds in different ways.
Do your research. There's tons of information out there on the internet. It's just a matter of being well informed.
Experiment. Something may look fun or good or whatever, but you won't find out until you actually....you know, do it?
Listen to your body. It's the best judge of whether or not something is right for you. No pain does not necessarily equate to no gain.
No one knows all the answers. There are new things developing every day, but keeping informed will let you make the right decisions for you.