How do you find your subcontractor bookkeepers?

I have a nice list of local bookkeepers from various directories, but how do I know which ones I can trust? How do you find your subcontractor bookkeepers?

Clearly the answer to this is experience, but how do you assess a bookkeeper's competence before you give them work? Is it the done thing to ask for references? I am wanting to build links with local bookkeepers with a view to outsourcing work, and I realise that the only way to judge the quality of their work is to give them something to do! At the moment, the only person's work I trust is my own (having had to pick up the pieces various times in the past when taking over jobs from other freelance bookkeepers/ qualified accountancy firms).

I was hoping the right person would just drop into my lap but this seems rather optimistic, and so I'm trying to be a bit more proactive.

Does anyone have any hints and tips?

Thank you!
Faerie Girl