Can they bid from plans, fast, and accurately?
A sub who takes two weeks to turn a bid, or who bids low and finds "extras" once they're in the walls, is a sub who's going to cost you time and margin. Send the same plan set to a few subs and time the responses — the ones who take it seriously as a business relationship respond seriously and quickly.
Do they actually hold phase dates?
The real cost of a slow plumbing rough-in isn't the plumbing — it's every trade behind them sitting idle. Ask directly how they handle scheduling conflicts across multiple jobs, and ask for a reference from another builder on whether committed dates actually held. This is the single biggest predictor of whether a sub is worth the relationship.
Full scope, or piecemeal?
Ground work, rough-in, and finish are three separate visits with three separate chances for something to fall through the cracks if they're not handled by the same crew. A sub who sees the project from trench to trim catches their own inconsistencies before an inspector does.
Are they honest about capacity?
A sub who says yes to every job regardless of their current load is a sub who's going to be stretched thin on yours. The subs worth building a relationship with are upfront about what they can actually handle — a few custom builds and ADUs at a time is a very different commitment than a 40-lot production schedule, and a contractor who's honest about which one they are is one you can actually plan around.
Permits and inspections — theirs to handle, or yours to chase?
Permit pulls, pressure tests, and inspection scheduling should be the plumbing sub's job, not something your PM has to babysit. Ask specifically how they handle failed inspections — a sub with a plan for that (rather than excuses) is one who's done this enough times to have a process.