Subcontracting Roof Replacements in Houston - How Do You Vet Subs?

Hey all, been doing roofing out of the Houston suburbs for about 10 years now, mostly residential shingle jobs after those hail storms we get every spring. Decided to start subcontracting out some of the bigger tear-offs to scale up without hiring full-time guys, but I've had mixed results with a couple crews who botched the underlayment and ended up with leaks on CertainTeed Landmark installs. What's your process for vetting subs - do you check their GAF certification or just run a background on licenses? I usually start with a small repair gig to test them, but twice now I've had to redo flashing work because they skipped the step flashing on valleys. Anyone have a good checklist or software for tracking sub performance? Looking to avoid any more callbacks this busy season.

For vetting subs, I always ask for their last three job references and check if they've got workers comp proof - saved me from a claim last year on a similar shingle job.