since this is the internet, when you click a link you don't always see what you want to see. so here are some tips:
1. check the users history.
if they are rude or un-trusted move their link down one on the safe scale
2. look at what people are saying about it.
check the comments or replies to see if you can find some clues as to whether the link is safe
3. check the link location.
if you roll over the link a little white box should show up on the bottom left corner of your screen with a link in it, if someone says:
this is a cool plane i made. this is the link for it: link you roll over the link and it says youtube, he's lying therefore the link is probably not safe.