For me, first and foremost the tell is whether the person has the right tools for the job. A tile installer who doesn't have a tile cutter speaks volumes. Second is some exposure to what constitutes best practices. An AC installer with two 110V circuit breakers in their hand instead of a single 220V breaker should be shown the door.
When dealing with installs and construction in this country, you don't need to be good at doing the job yourself, but you do need to know how the job should be done and what materials should be used. You have to supervise everything. 5 minutes of inattention is more than enough time for someone to really bugger up a job.
More and more, I'm opting to pay for transplanted trades people. No guarantees, but at least they usually know how they should do the job and I don't have to ask them to put their hands in their pockets until I come back from the bathroom.