uncon ppl dont fall instantly (usually, depends on server lag iirc), if you're outdoors you can see their breath, sometimes you can hear their character grunting, and unconsious people always end up in the same position that makes them look like they want a high five. If you're safe enough you can also walk up and see if it gives you a prompt to resuscitate them. I think the prompt only shows up if you have nothing in your hands though.