Common sense works very well everywhere except in real life.
If the most popular girl walks up to Social Reject X and asks for favors, but never talks or hangs out with Social Reject X, then duh. Social Reject X is being used.
If the most popular girl walked up to ME and asked for favors, rarely talked to or hung out with me, but always gave excuses that seemed valid enough without a pattern to them... I'd feel like a champ for a few months. Eventually, I'd realize that I'd been acting like a chump descended from chimps.
If you're really worried about being manipulated or used, the only solid indicator would be to keep track of what you did for them, and what they did for you. If there's a significant imbalance (make up your own rules for this, I don't care enough to) then someone is getting screwed. The next step is talking to the person. If you're screwing them, ask them what they need done. If they're screwing you, ask them to do something for you... Politely. Don't ask me how to do that politely - I haven't the foggiest idea.
If they refuse, give irrational excuses, or otherwise attempt to dodge the issue, I'd move on.