With the PMP credential, you are guaranteed that, at one point in his/her relatively recent past, (s)he took and passed the PMI test. All else are uncertain.
@Hasan in response to comment: I believe I answered the question and I meant exactly what I wrote. The PMP test is knowledge base, psychometric test that touches a little bit on skill, based on basic PM principles and practices as indicated in the PMBoK. A PMP certificate guarantees that the person has passed the test and had at that time the basic knowledge to pass it at the then passing score. It guarantees nothing else, including whether that person has retained what (s)he learned to pass it or that (s)he really understood it, the latter indicative of boot camps. To maintain the PMP, you are required to obtain PDUs, which I suppose provides a degree of confidence that the individual is maintaining or continuing to learn; however, there is really no oversight in terms of knowledge acquisition, master, and retention with those PDU classes. So the degree confidence you might have is further degraded.
There has been no quality examination that correlates the performance to the PMP-certified practitioner. None. There is no evidence that a PMP performs better or worse than the non PMP. Thus, you can generalize nothing from that, either, meaning you have continued uncertainty.
At the end of the day, a PMP guarantees only that a test was passed.