Diagnosis online is just like in person. A lot of therapists will avoid it either way, and different therapists can have different opinions too. Diagnosis isn't all it's cracked up to be. It's really just a label that attempts to describe your symptoms and put them into a category. Most therapist will discuss diagnosis with you, at least after enough time to get a feel for what your symptoms are, if you ask.

Online can be a good way to get started with therapy. You can get support and insight that way, and either way you get out of it what you put into it.