There are a few things which will tell if your painter knows what he or she is doing.
Preparation. If it is walls or wood work 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, not trusting just your eyes.
All lines cut to ceiling, skirting, walls and windows should be sharp, with no brush strokes around plug sockets or light switches.
Drop cloths and dust sheets should be used plentifully, to ensure proper protection of your home and valuables, from dust and paint over spray from rollers etc.
The colour on the walls should be even with no streaks or difference between cut in and rolling.
All gaps should have been caulked between skirting and architraves.
Raw wood should be knotted before priming, a step often over looked, and one will not know till several months later, when brown marks appear on your wood work.
If these steps are followed then you have the right people for the job.
And remember a professional painter (or what ever trade) should always clean up after themselves.
The painter himself / herself should also be of neat appearance both them selves and their van, this its self can be a good sign that you are on the right track.
Last but not least check for insurance very important.