Against one opponent, how do you play this? For example, last night I had JJ in MP and raised, the BB called and the flop came J 10 4 two hearts. He checked, I bet out he folded and I won a small pot. Is it safe to slowplay here or no? Against more than one opponent I'd play JJ very fast but against one isn't it a lot less likely you're up against a draw?

Generally I'll play it very fast, even against a single opponent, I'll rather him fold then catch a draw and take the pot away from me.

I would say it depends on a few things though :

How much did you raise pre-flop, whats his table image, whats your table image. If he's likely to call a modest raise with 98 for instance, then you have to play this very fast. If he had anything besides a draw he'd likely stay in there with you, or maybe come over the top, sounds like you just got unlucky this time and he had nothing.

Its always better to win a small pot then to loose a big one in my book.

As of lately I am now on the flop betting out 20-35% of the pot to see where I stand. As you just said, more then likely the other person will fold, if he is calling, then I have an idea what his cards probably are.

This kind of hand should be about extracting the most valuable possible. In general, flush draws are the easiest ones to spot, so your opponent's implied odds are very small, making it possible for you to bet less. As long as you are not giving him proper pot odds, and are able to fold if he does hit his draw, betting around 1/2 the pot or a little more to induce a call will have a greater long term expected value than ramming and having him fold out every flop.

Some more info would help for analysis purposes. ie blinds, chip stacks, ring game or tourney, if tourney position and average stack as well as ITM would all be useful. Without this info I'll give you one general concept to consider. When you raise A J next time and hit top pair and kicker what do you want villian to do. Unlike your example were you have 1 card improvement to the the nuts versus a straight or flush with the set. Chase the draw or fold. I think you bet this hand the same as your preference would be to have villian call top pair and kicker or fold. This allows you to disguise your sets totally from top pair and kicker postflop/preturn. I like small ball so I'm OK with very little action. Nothing wrong with trapping as long as you can let the hand go if the draw comes good. A Jumbo draw(ie 15 outs less redraws to your boat) is priced in up to a pot bet postflop. I like that bet(pot) postflop in ring game poker particularily to define likely pot odds when hitting top set. A miss of the potential draw on the turn is an all in everytime post forth street on pot odds alone when in a ring game. You defined a winning net strategy by betting pot on the flop if you get called. And a non Jumbo should fold but doesn't you have a net gain overtime post flop rather than turn. Even better.

I'd definitely not play it slow here. Even if he folds, you risk the chance of him having pocket connectors and then getting his 5th heart on the turn or river, depending on how slow you're playing it. Then, it gets real hard to lay your hand down due to the fact that you have a real strong hand and that you want to prove to yourself that you didn't make a mistake.