Perimenopause starting point?

I understand that certain medical procedures can kick-start perimenopause. But I've read people who post things like "I've been in [peri/]menopause for xx years..." and without something significant to indicate the starting point, how do you know? My symptoms started I have no idea when. For several years I've had issues that at first I attributed to other things. I've finally come to the conclusion that it's perimenopause but how does anyone really know? Furthermore, to what extent does it matter?