Completely agree with Kristina, I've seen the same assertions - assume that whatever difficulties the child currently has, they will continue just the same in your home.
Questions for the FC about the child, I definitely agree with "What were they like when they first arrived?" and how much progres has been made
I made a rather extensive list once for a prospective mum, based on things I asked DD2's FC's. I'll copy the relevent questions! This mum was seeing the FC of a much older boy, so ignore the 'he' throughout it, and ignore anything that's not relevent, but I suggested these kind of questions:
The worst behavior he has shown was?
And the worst his behaviour gets now is...? How often does this happen?
What does and doesn't work when trying to discipline? How does he react to discipline?
What works best at diffusing/ dealing with tantrums?
How long does a typical tantrum last? How often do they happen? What would trigger a big tantrum?
How does FC anticipate he will react to a move to adoptive parents?
What things make him angry/distressed? Can you always tell when he is feeling very stressed/distressed/angry or does he bottle it up?
Does he let himself be comforted by FC's when this happens, or not?
And conversely of course, what tends to make him feel happy and calm?
What does the FC think about the childs ability to attach and form a close relationship with parental figures?
What kind of environment does he do best in/is happiest in?
How much routine does he need? And what happens when the routine is deviated from, is he a 'go with the flow' type person, or would he react really badly to that?
How much supervision does he need every day, and for what?
How many friends does he have? Is he good at making friends? And keeping them? How does he behave around friends, and play with them? Does he get on better with kids his own age or gravitate to younger/older children? - (This will tell you things about the childs social and emotional development)
How would you describe his personality?
I asked DD2's foster mum what she thought DD's emotional age was as well.
With siblings, I'd be asking questions about their relationship with each other - how they behave towards each other, what the FC thinks of their relationship etc
Teacher -
Classroom behaviour
Behaviour compared with their peers?
Friendships from a teachers perspective, how child gets on in the playground
How their work compares to their peers, how fast they learn and how they are progressing
What support is needed in the classroom, what teacher anticipates would be needed in a new school