“Can a psychotherapist diagnose psychopathy?”

While a psychotherapist (master's level counselor) is able to diagnose psychopathy, I would recommend a psychologist (PhD or PsyD) for diagnosis and evidence-based treatment.

Psychopathy isn't really an official diagnosis, but rather a label for a set of enduring characteristics and tendencies, and is on a spectrum of mild to severely malignant. In my experience, they don't often show up in psychotherapy and so don't get an official diagnosis, except perhaps in correctional facilities. Some of them are very public and reveal themselves very clearly, perhaps more clearly to a psychotherapist, but the conclusion is more in the nature of characterization than a diagnosis. If you believe you are involved with someone with 'dark' personality traits and behaviors, a psychotherapist can be helpful in figuring out what, if anything, you can do.
I hope this is helpful.

Only those such as myself, who have experience working with psychopaths.

A good treatise for this question is:
https://www.verywellmind.com/an-overview-of-psychopathology-4178942