How do you actually vet a recruiter before committing to a search together?

I've been on both ends of this and it's hard. 

How do you actually vet a recruiter before committing to a search together?

When you're evaluating external recruiters, the pitch is almost always identical. Deep network, quick turnaround, great relationships with candidates, full focus. None of that is verifiable upfront and by the time you figure out someone isn't a fit you've already lost weeks on a search that went nowhere. 

I'm curious what signals people actually look for. Not what recruiters say about themselves but the things you can observe or check that actually tell you whether someone will be useful before you hand them a role.