I haven't done affiliate screening for a few years, but here's a couple resources.
Besides the obvious things you want to screen out like affiliates that have adult content, gambling or whatever - one of the main things you need to check is whether it's a fraud affiliate. So you need to try to determine if the affiliate is really who they say they are.
I had a big Chinese fraud right joining a bunch of networks and programs pretending to be Linda with 5 Star. Used all my Whois info so had all my personal contact info and used it. So unless you call to screen affiliates it can be hard to tell who's really applying.
These should help:
I need help Fighting Fraud
Phony Affiliates Prevention Guide for Managers