You can help your chances of getting lucky, but there are no guarantees until you feel or see it.
The way to help your odds are by:
Height - The taller the guy the better chance there is he has a big one. Many don't believe a correlation between height and penis size exists, but this has been studied indepth at Thundersplace and there is somewhat of a connection. While the overall average size between tall and short guys is only 1/4" difference, the outliners (big and small dicks) showed a significantly higher occurance in relation to height. Tall guys twice as likely to have a big one.
Bulge - The bigger flaccids are more likely to be big when erect. Many don't believe there is any difference upon erection, but this has been studied indepth at Thundersplace and there is a correlation between flaccid and erect afterall. For instance, 3" flaccids were found to become 5.75" erect (overall generally speaking after erect numbers were added together and divided by number of guys), but 5" flaccids became 7.25" (generally speaking again). Some 3" flaccids can become bigger than some 5" flaccids upon erection, but the odds of becoming big are on the side of longer flaccids. Also, concerning the "grower" vs "shower" discussions that occur from time to time, in which so many seem to think everything equals out, notice that the 3" flaccids did grow more, closing the 2" flaccid gap (5" minus 3") by 1/2" making it a 1.50" difference (7.25" minus 5.75"). So there is some truth to the common belief that "growers" grow more than "showers", but it's mostly a false assumption as in general they do not catch up. Also, the longer dicks tend to have more girth, as the study indicates a strong correlation between length and girth, though of course once again there are plenty of exceptions and therefore no guarantees.