In the affiliate game it's often easier to spot "signals of cluelessness" than "signals of quality."
If the merchant gets ordinary things right, avoids obvious mistakes and seems like they'd be a good fit for your traffic, they're worth testing.
"Just trust us" ... Forget trusting, stick with testing. There are many things on the merchant end that the affiliate has to take on faith, and I don't see any way around that. You either trust or you don't. If you trust, then you test. The results are what matter. The most reliable metric is to promote the merchant for a while and decide whether you like the paychecks you're receiving from the merchant.