Look at completed projects of your size and complexity
Photography lies. Walk a completed build of similar scope if you can, at minimum, talk to the owner. A builder who has built two cottages your size is not the same as a builder who has built fifteen.
Ask specifically about lakefront, four-season, custom work. A builder who does production homes in town is not equipped for your build.
Understand how they're paid
Ask directly: is there any markup on trades or materials? If yes, you're under a GC model. If no, you're under a fee-only CM model. Neither is wrong, but you should know.
Ask how change orders are priced. Marked up or passed through.
Ask whether you'll see every quote and every invoice. The answer should be yes.
Meet the daily site lead, not just the principal
The principal sells the project. The site lead runs it. They are not always the same person and they are not always equally capable.
If the principal cannot tell you the name of the site lead at the first meeting, that's a signal.
Warning signs that should end the conversation
Reluctance to share recent owner references.
Refusal to detail how trades are marked up or passed through.
A fixed-price contract on a custom build with no allowance schedule.
Vague answers on backlog and start date.
Any version of 'don't worry about the details, leave that to us.'