You'd think this but I don't know. In a small town, maybe word of mouth would hurt a persons business if they did shoddy work, but I found in a larger city contractors don't care too much because with 80 new people a day moving into the city, there is always a person out there ready to either not do any research or just assume the reviewer was a bad apple. I've written a few negative reviews with specific facts and look back later and there are 20 new reviews, 14 of them negative with the "I should have checked reviews first.." as the lead off sentence.
Red flag 1: The guy who is billing you isn't the guy doing the work. Don't hire a trade broker with 'his crew' who aren't tradesmen enough to be out doing their own thing. I've never seen a crew who work without the contractor present do a good job. It has never happened. They will deliver 'good enough' which isn't.
Red flag 2: No trade specific tools or a whole lot of cheapo brand tools. If you're in the game for real then you have good gear, all of it, and it looks worked.
Red flag 3: They're the cheapest. I know that the only thing that counts right now is money. Later, the only thing that will count is quality. Never ask a contractor to beat the other guy's price. Once I hear that, I won't even do the job for what I quoted. Not everyone can afford the Taj Mahal, and that's fine. Some corners you can cut no problem, some you barely can, and other's you shouldn't ever. An experienced tradesman knows which are which. The cheapest guy is probably going to cut them all. Hire him and you're complicit.
Talk nuts and bolts. What he proposes to do, where and how. Timeline. Materials estimates. But have your project clearly defined and have this discussion once. If you don't even know what you want then how the hell am I going to put a price on it?
Asking for references is fine but don't repeatedly seek assurance that the work will be good quality. You'll just seem like a fretful nut. If the guy refuses to do something that will save you money because it's shoddy, that's a good sign.