That's an excellent question and there's really no easy answer. Pretty much we're trained to use our instinct to blow that whistle at the "first sign" of a foul or violation. Sometimes we get in the habit of anticipating a foul when the play is developing into a certain circumstance and we blow the whistle but the foul actually doesn't occur. I think the best thing for you to do is just practice waiting, practice waiting and then practice waiting some more. Just don't wait too long or you'll get the reputation of having a "slow whistle" (which, I guess, is better than having the reputation of having a "quick whistle"). Also remember that if you wait "too long", the play will continue when it shouldn't and other things may occur. The ability to get the whole picture will come with time and is based on the advantage/disadvantage principle.
Remember, all contact is not a foul and sometimes a play looks kind of "goofy" but, by rule, it's not a violation.
Just think, if it didn't require any intelligence to officiate, Billy Packer would be doing it.
