Here are two considerations:
1) You will want to have a smooth surface. Any brush marks or other irregularities will telegraph right through, regardless of how many coats of new paint you put on. You may be able to sand it down smooth without stripping, but...
2) You will want to be sure that your two paints are compatible. As a rule of thumb, you cannot put a "harder" paint over a "softer" one. If the paint was put on sloppily, I would be figuring the person probably did not go all-out on an expensive "hard," paint, but you can't know for sure unless you have the old can of paint in hand.
For some paints (usually two-part paints), they have a way you can test the current paint (should be mentioned on website or in technical literature). For example, one test is to score the existing paint, soak a cotton ball or rag in the solvent used for the new paint, tape it on over the scores, and wait 24 hours.
In the end, you have to do what you are comfortable with. For me, the prep work involved in painting
(as compared to the quick little step of actually painting
at the end) is enough that I don't take chances. If I don't know, I take the conservative choice.