Before you enter photographic competitions you don't need to look into some sort of crystal ball, but you should always read the Terms and Conditions of entry.
Some, not all, competitions have conditions that effectively give away your copyright! Some actually state that by entering the competition you are signing over your copyright to the organisers or maybe they are claiming the right to use the photo for ever for ‘free’! You won’t even be paid a penny.
Submit your photographs to such competitions and you lose valuable rights to your photo. If they have claimed the copyright it's no longer yours, you could actually be sued for using it in future. Or maybe they have claimed the right to use it freely forever, to make money from it by licensing it to others, but you'll never see one penny of that money earned by licensing your photograph.
The good news is that a lot of competitions are safe to enter without losing your right to own the copyright. An organisation called Pro-Imaging is fighting the bad practice by some competition organisers by investigating the Terms and Conditions of the competitions. The ones that are safe to enter are listed on their Rights On List, and the ones to avoid are listed on the Rights Off List.
(If the competition you are thinking of entering is not listed, use their contact form and they will check it out for you!)