From my (luckily) limited experience :Or spot a cheapass contractor.
- They are ready to start to work very quickly or even right now,
- They are very talkative and easy going and will try to get you to sign (as opposite to a good workman who is never missing work)
- Promotional material or stands, ads, flyers, and the like may be warnings since not only it means they need it to find jobs (see previous point) but also they spend time and money on that and not work-related purchase.
- If they offer to work for free or cheap because of state subsidies they'll do a shitty job guaranteed.
- Gut feeling is a good enough reason to deny a contractor.
The best way to find contractors is to know other people who employed them and are happy with the result. Otherwise get several quotes and compare them.
Otherwise learn to do it yourself if you're handy. Tools are expensive but it will still be cheaper than paying someone, and the result will be better than a cheap contractor. But it takes time, lot and lot of time.