If you get offered right on the spot, you could always ask them how quickly they need an answer before you worry too much about it. They might be early in the interviewing process themselves. Worst case, just ask them for a little time to make a decision and see what they say. You should be able to read their response and act accordingly.
I may get blasted by others, but if you need to accept one internship and back out of it for something better, I wouldn't lose sleep over it (unless your field is super tight knit like CG said). It's a jaded outlook, but I feel most emloyers are looking out for their best interest. You are entited to do the same.
You probably ruin any chance of working for that company later on, but if you aren't willing to take an internship with them, your mind about them is already made up. They are a fall back and not a destination for you.