How can you learn when you can/can't do something?

I read a post a few minutes ago about making decisions and knowing when to fight and when to get the hell out of there. It talked about staying alive, giving up towers, etc. But what if you can survive and get away with killing them? Let's say you have your 2 bot laners bot with a tower, and the enemy bit laners pushing thr wave, about to kill the tower because the other bot laners were low. I mean you should give it up to live, but what if you could have killed both of them and kept your tower? You just gave up a tower and 2 kills. I mean. It would take serious outplay potential, which I don't have. But when should you know if you can win a fight or if you need to back out to survive? (Sorry if this didn't make much sense)