Interview them for the job.
Do they offer you a copy of their contract of their terms and conditions?
Do any of them have an eye on the market, mentioning a quick sale price?
Ring them all up and ask them as if you were planning on buying in the area. How do they deal with you? (ok, get someone else to do it, or use a relatives address and name) - but this way you know how they deal with people. Doing this first might save you time interviewing them!
Ask them how many people they have on their books wanting houses like yours.
Chances are they all get the same customers - so you want someone who's efficient and easy to deal with.
What tie in did they all ask for? What fee? Will they negoiate?