When you are not so big a favorite, it is even more important to bet rather than give a free card. With a modest pot you should come out betting even though you expect everybody will fold because you can’t let somebody with, say, a lone get a free shot at a higher flush.
You might not want the person to fold when you bet, but making him fold is better than giving him a free chance to outdraw you. (The only time you might check your flush is if the pot is so small you expect to gain more through deception. Thus, if no spades fall after the flop, your profits on later bets are likely to be considerably larger than what you would gain by betting on the flop. However, if another spade does come, you have to be prepared to fold.)
When you have a chance to bet and you have a decent hand, especially a hand you think is the best one, it is almost always correct to bet. Nevertheless, it is still better that you bet and force your opponent to fold than that you check and he check behind you. By checking you are giving your opponent a free shot at beating you, a chance he would not have if you had bet.