Just pay attention to the signs above the FP booth. They'll tell you when your fastpasses will come due, and you can compare that to the sign above the Standby line to see if it's worth grabbing one.
You have an hour to use your FP, so it's not hard to make it back in time if you really want. There's even a 15 minute "grace period" printed in tiny letters on your ticket (look for the initials GP followed by a time). If you decide you don't want it, you haven't lost anything other than the two minutes it took you to grab a FP.
You'll be able to get another FP when your first FP comes due... assuming it's within a reasonable amount of time. If you get a really popular FP and it's return time is way out in the evening, don't panic. You don't have to spend the whole day without another FP. Your window to pick up a new one will be within... I think it's two or three hours. I'm not sure, but it's not that far out. You'll see the time you can get a new FP printed on the ticket.
The way we do it is we look at the length of the Standby line and decide if we really want to jump in. Sometimes we ask a cast member if it's accurate (it isn't always, and they generally know). If it's too long for our taste, we grab a FP and move on to the next ride.
No stress!
It also helps to pick up a guidebook, so you have some idea which rides are the most popular, and which don't require FPs at all. Folks here can help you prioritize. (For example, you generally don't require a FP for Mickey's Philharmagic. Ever.)