Good luck. IMHO be led by foster carer, it is there home (presumably) and even if not they know the child so let them talk as much as they can before you ask too many questions. Unless it is all totally unhelpful, then you can direct them a bit!
Be aware what they say may be true for little one at their home but not at yours! our son ate everything and went to bed 7.00 to 7.00 in a darkened room. He now eats very pickily, goes to bed later, with lights on and gets up at later.
Either he has adjusted to us or he has felt able to let his real (fuss-pot) self out!
Pay attention to and make notes of everything you feel is really significant. EG medical stuff, next appointment with hospital, GP, ears/eyes/whatever, and behavioural stuff.
IMHO just get a feel for how little one is and make good friends with foster carers and swap email or mobile numbers if allowed so you can ask stuff later, IMHO.
There is a long thread on here about it, started by me about a year ago. I can't find it at the moment but maybe you can if you have more time.
There is also this shorter threat at www.mumsnet.com/Talk/adoptions/a2247326-Meeting-foster-carers-tomorrow-what-do-we-ask
There are more if you search.
Good luck.