That’s pretty much what I have done and gotten a variety of responses. It is rewarding to work with the ones that appreciate the help. Had one kid that had pretty much signed with a well known college for lacrosse and was working with the high school coach at school but when he would come to the gym he would workout alone. Started a conversation one day because he had that lost look on his face. Helped him out with a few things. At the end I said I’m not looking to train you but please feel free to ask me anything, and he did. I continued to help and he listened and made some great progress.I've spent a lot of time in gyms and have probably trained well over 1000 people how to squat, pull, and lift properly (not really that many as it was never my long term career but just saying). A lot were athletes so they needed to listen and had some motivation but plenty of normal people too and kids.
I very rarely offer unsolicited advice. Kids learning and working is where I'm more likely but even then I put out a feeler first. I do not want to waste my time on someone who thinks they already know everything or wants to tell you what they know (cause I already know by watching that they know jack shit). Also the single worst guy in the gym is the one trying to help or instruct everyone...invariably they know nothing and just run their mouth constantly.
Anyway, watch, put out a feeler to test receptivity. If so and they WANT to learn, then help. Kids are rewarding to help. You really help someone they are thankful. Hell, I wish there were people who knew what they were doing when I was younger. Damn, I had to learn everything the hard way.
As far as form...I also tell them to get a copy of Starting Strength. Best technique book I know of. As far as goals...progressive resistance. Gradual increases add up hugely. 5lbs a week X 52 weeks is crazy fast.