Help with Loose Passive Players?

[HobbyHorse]

I know, I know, most of you are already salivating at the mere mention of loose passive players, yet I find that I am having some difficulty trying to figure out where I am in the hands because these loose passive players rarely bet/raise...they just call...which is fine if I have a drawing hand, but in other situations, I find that I am uncertain about what they could be holding...

For example, here is a hypothetical situation: live full ring NL 5/5 game, Hero has around $300 and villain the same. Say Hero limps in from BB with 9 [img]/images/graemlins/spade.gif[/img] 10 [img]/images/graemlins/heart.gif[/img]. (For simplicity's sake, say there are only two callers - Hero and Villain. Normally, there would be 4+ callers in this game)
Flop comes 7 [img]/images/graemlins/diamond.gif[/img] 8 [img]/images/graemlins/diamond.gif[/img] J [img]/images/graemlins/diamond.gif[/img].
Villain (in MP) checks, Hero bets $15, Villan calls.
Turn comes 7 [img]/images/graemlins/club.gif[/img].
Villain checks, Hero ???

Now, obviously this would be a great flop for me since I flopped the nut straight, but I have such difficulty trying to figure out what the other players could have because they are so passive. I have seen players in this game not bet their flushes until the river, and then bet them very weakly. And because these players are so loose, it is equally likely that they could have limped in with J7o. They rarely bet straight out other than on the river - but they will continue to call down on all streets. And their bet sizing is messed up too because they rarely vary their bets with the size of the pot. They'll just bet the same amounts on the flop, the turn, and the river. I start second-guessing myself because I continue to bet and they just call and call and call. In this situation, it feels like my bets are more like checks and I'm gaining no real information.

Any advice? Sorry for the remedial nature of the question...