Any other ways to highly suspect someone's on the flush draw besides...
1) there's 2 on the board after the flop or 3
after the turn
2) someone keeps betting small or calling small bets on flop and turn
3) or possibly bets the pot with 3 on the board
(meaning they may have the A of the suit)
I think you've nailed the most common clues. Alot of players will bet out on the flop if they are on a four flush to throw off opponents of reading them for a flush draw. Basically, the easiest way to tell is if someone just seems to "hang around" a hand. To counter-attack that, I like to throw a raise somewhere in the mix if I'm on a draw. This does a couple of things for me. First and foremost, it throws off any kind of read on me. Secondly, it builds up a pot if I do hit. In this case, if I have the nut flush and the board isn't paired, I like to check the turn and trap someone into betting into me. If this happens, I'll either check raise or call and check raise the river depending on position and opponent. Sometimes, raising on a draw will actually push someone out who's just trying to get info as to where you stand in the hand. They may be on a draw themselves and decide that it's just not worth chasing.
It's a lot easier to figure out in limit...9x out of 10 the flush draw will raise your top pair or the like to buy a free draw at a flush. I always play back at that person if I have a set or top pair top kicker. If they slow down, then I know they are on a draw. If they don't, they still could be on a draw and jacking up their odds to call the turn, but that takes a real aggressive player and is rare.
In NL, it's much more complex. Sad to say, but I think the only way to find out is to make a healthy opening bet and see how they respond. MOST good players will not risk a significant amount of their stack on a draw, but you really have to look out for the "go big or go bust" players, because they will jam you right back and then you better have no less than two pair to stay in.
Now if you are talking live play and looking for tells, watch how they look at the board. This has only worked for me once but I remember looking for this and seeing it.
Watch closely how they respond when you act. If you put a flush draw to a decision on the flop, there is a natural human instinct to not trust what one has seen at first. They're pretty sure what they have in their hand (and if someone goes to their hand to look at their cards, don't fall for that--it's a pure fake unless the player is a bonehead), but the board has three heretofore unknown cards and before committing a goodly number of chips to the pot, a person not in total command may want to make sure that the spade he saw is not really a club.
This worked for me when the flop came 2hJhKd, and I made middle set. I bet out, and the guy took a quick scan and his eyes moved perceptibly to take in the whole board. He raised. Kh hits the turn. I checked, he bet out, I raised, he called. Got him again on the river too. I understand this hand plays the same way without the tell, but I remember seeing that moving glance and giving myself a pat on the back for actually picking up on it, which I usually can't do.
