The percent chance that a specific opponent has a better pair is as follows:
Take the chance that his first card is better. (Eg: If you have pocket 2's, there are 48/50 cards that are better.) Multiply by the chance that the second card pairs the first, (Always 3/49) and you get the odds of them having a better pair. Multiply by the number of opponents to get the chance that anyone at the table has a better pair. I'll leave the chance that more than one person has a better pair as an exercise for the reader.
But I'm not sure how useful this is at the table. If they are already raising, then the likelyhood that they have pockets goes up. Since the cards have already been dealt, the chance that they have a better pair is either 100% or 0%. At this point, I think your read on the opponent is more important in deciding whether to shove.
(Disclaimer: I think the math is correct, but I don't guarantee it.)