A big warning sign was that my employer interview great candidates, make offers, and then the candidate didn't accept. The problem was that we weren't paying competitively.
I tried to express this to upper management, and then instead of fixing the problem, they just gave me a raise. (From below market rate to below market rate but able to start putting money in the newly-offered 401k.)
Shortly afterwards, I interviewed at one of our major competitors. The next day I looked at one of the founders and said, "if there's a chance to exit via acquisition, we need to take it."
Turns out there was an acquisition deal in the works, but we couldn't know due to how US law works. One of the higher-ups asked me to investigate "a bug," and when I looked at her logs, all I saw were references to an upcoming acquisition. I then knew to stick around and give the new owners a chance.
There's a lot to be said for sticking through a few months of uncertainty when it works out to be a great job in the long run.