There are a few things which will tell if your painter knows what he or she is doing.
1. Preparation. Whether it is walls or woodwork that is being painted, the area chosen should be smooth when finished. It is a good idea to feel the surface by running your hands on it, this is a sure way to know that the area was rubbed down before painting.
2. All lines cut to ceiling, skirting, walls and windows should be sharp, with no brushstrokes around plug sockets or light switches.
3. Drop cloths and dust sheets should be used plentifully; to ensure proper protection of your home and valuables, from dust and paint overspray.
4. The colour on the walls should be even with no streaks or difference between cut in and rolling.
5. All gaps should be caulked between skirting and architraves.
6. Raw wood should be knotted before priming, a step often overlooked, and you will not know till several months later when brown marks appear on your woodwork!
Remember a professional painter (or whatever trade) should always clean up after themselves.
The painter him/herself should also be of neat appearance – both themselves and their van, as this itself can be a good sign that you have made the right choice in decorators.
Last but not least checking for insurance is very important.
If these steps have been followed then you have the right people for the job!