How do you know if you have histamine intolerance?

I've seen a lot of long haulers saying they have histamine intolerance or MCAS but I'm not sure if I have histamine intolerance myself. I don't feel any worse after eating high histamine foods. I've been taking both H1+H2 antihistamines for a couple of months but I don't notice any effect. Does this mean I don't have a histamine intolerance? Or do I have to go on a low histamine diet for a number of weeks to see if there's any difference?