Telling real betrayal from jealousy that is out of proportion
If your partner has been unfaithful, that is a real problem with real evidence, not a symptom. If the jealousy runs on doubt that no evidence settles, it is treatable. Assessment tells them apart.

Before anything else, I want to say what I will not do. I will not pretend a genuine betrayal is a symptom. If your partner really has been unfaithful, that is a real problem with real evidence, and it deserves an honest conversation rather than a treatment protocol. Part of assessment is telling those two situations apart, and I will give you a straight answer about which one I think you are describing.
That said, most people who come to me about jealousy already suspect the answer, and the suspicion is why they came.
The evidence test
The simplest distinction is what evidence does. A real concern responds to evidence. If you have found messages, been lied to about where they were, or been told directly, the concern is evidence-based, and it will resolve, one way or another, when the evidence is dealt with. What follows may be painful, but it is a relationship problem, not an anxiety problem.
Jealousy that is out of proportion does not respond to evidence. The clear phone does not settle it, and neither does the plausible account, or years of finding nothing. If you have checked repeatedly and found nothing, and the doubt is still there, reformulated, the doubt is running on its own engine rather than reading the evidence, and the engine is the one that runs OCD.
The proportion test
Another sign is the gap between what you know and what you feel. Many people describing pathological jealousy to me say, unprompted, that they know it is out of proportion. Knowing that means the rational part of you has assessed the situation and found no threat, and the jealousy has continued anyway. When knowing does not touch it, evidence was never what it was about.
The compulsion test
Look at what you do. Checking their phone, questioning, testing them with trick questions, monitoring their face when a name is mentioned, searching the social media of strangers, mentally reviewing an evening for what you missed. Each brings relief for about an hour, and then the doubt returns needing another check. That pattern, relief that never lasts and checking that never terminates, is the signature of a compulsion, and compulsions feed the doubt rather than gather evidence.
When both are true
Sometimes there has been a real betrayal, in this relationship or an earlier one, and the jealousy has outgrown it. The betrayal was real, and it is over, and the checking has continued for years past the point where it could find anything. That is a case where both things are true, and the treatment addresses the machinery while acknowledging the history. Where a past betrayal or abandonment is driving the present, EMDR can take the charge out of the memory that keeps the doubt alive.
What assessment looks like
We start with an assessment session or two, mapping the doubts and every compulsion attached to them. I will ask what you have found, what you have checked, and what happened when you did. I will ask about history, yours and the relationship's. And I will tell you plainly which picture I think fits. If it is a real problem, I will say so, and I can point you towards couples therapy, which is a separate conversation from this work. If it is the machinery, we treat it.
What therapy will not decide
Therapy will not tell you whether to stay in the relationship, and I would be wary of anyone who did. What it can do is give you back the ability to make that decision from a settled place rather than from the middle of a doubt spiral. Many people find that once the checking stops, the question of the relationship becomes much clearer, sometimes because the doubt fades, and sometimes because a real problem that was buried under the jealousy finally becomes visible.
If you are not sure which situation you are in, there is more on how I treat pathological jealousy, and the first conversation is where the distinction gets drawn.