Ideally, you want a variety of older and newer projects, and you especially want to check on a very recent project, such as something completed last month. When you do get the references list, don’t take the information for granted. Take a few minutes to check these references. Ask these homeowners about the experience while services were provided and also how well the job has held up over time.
I agree with JHs. If there is no new color on it at all, just talk to the painter about it, if it was an honest mistake (which can happen very easily while painting) your painter should have no problem fixing it. However, in my years of experience, it is not unusual for 2 coats of door paint (good quality) not to cover very well at all. I once painted a red door 7 times, plus a tinted prime coat before I found the door to be a solid color.
His team is very meticulous, very careful, very clean, and very courteous! They're also very accommodating. For instance, I mentioned a couple spots under some angled siding where I thought a little extra filling was needed, and they obliged without question. Same thing with a request for a stain-proof primer under a balcony. Doing these kinds of little things without question or extra charges is really nice!