If I'm out of position and I have a feeling my opponent is drawing to a flush, and it gets there, I check. If I'm faced with a medium sized bet on the river, I sometimes call, sometimes fold. You just have to trust your instincts, I guess.
Realize that at $50NL (IMHO), people are going to draw to the flush no matter what. I've seen people call an allin with a flush draw MANY times (huh?). So, you probably can't stop them from going for it.. just charge them and don't pay them off when they hit (hence, Maja's check).
at 50NL; do me a favor and count how many check call the flush draw, turn completes it, they check flop bettor leads again, min raise by flush chaser. Its almost 90% accurate. Just what I have noticed. Its unreal how many flush chasers pull this move out.