You need to do a bit of simple testing and play acting:
Prepare a simple A4 questionaire with 10 questions on it, and maybe 9 photos of houses on it [tick the house/price you like photos] and give them some story about "How much you want to work with them", in order to do so you need to know their needs, wants, budget, and hopes. Leave space for them to add loads of information on it and ask them to bring it back to you later in the day or post it back to you. In effect your saying - without saying it, prove to me you really are interested, you see 8 out of 10 won't bother - SO THEY REJECTED THEMSELVES.
I call this the go for a coffee break and come back later screening method, others might have another name for it.