How is PMS diagnosed?

The simple answer to the question is, there is no specific method of testing or diagnosing PMS. Because there is no established laboratory test for the condition and none of the symptoms are unique to it, there is no specific method of diagnosis either.
Consequently, it is likely that for any woman who suspects that she has PMS, your doctor or other medical attendant will ask you to keep a diary covering at least two menstrual cycles to establish whether the cited symptoms re-occur on a regular and predictable basis. Armed with this information and assuming that some or all of the symptoms of PMS are present, your doctor is likely to considerably closer to confirming that PMS is the problem.
However, he or she will also need to eliminate other potential causes of your symptoms as well before finally confirming that your problem is indeed premenstrual syndrome. This is important because many symptoms that might be associated with PMS such as depression, stress and anxiety may be a result of PMS but they can be caused by many other physical or psychological conditions as well.
The key here is likely to be the presence (or lack of it) of the symptom outside of the luteal phase.
If depression, stress or any other possible symptom of PMS is seen to be present throughout the month or outside of the luteal phase, it is considerably less likely that it is a symptom of PMS. Consequently, there may be a completely different explanation for that symptom.