In general, feeling an attraction to the priesthood is a good indicator that one is called to become a priest. Either that, or at the very least, feeling that becoming a priest is what he is called to do.
If a man feels that becoming a priest is what he is called to do, or even just has an interest in the priesthood, it is a good idea to talk to a priest about it. A priest will be able to give better advice than us on the internet, because he is better able to answer any questions, concerns, etc.
Another great resource, which was mentioned above, is the book To Save a Thousand Souls.
Prayer, time with Out Lord, and confession are invaluable.