Some of the memory foam exercise mats, or stadium seat pads, will retain dimples in them where your sit bones were. You can get close enough to tell if you're in the ballpark. I've got a few BG saddles that are 143 and one that is a 155. I might be able to convince myself one fits better than another if I had paid real big bucks for them, but I really can't tell the difference with the end that matters. I've never had the wider one, almost 170mm I think, to compare. The narrow to the wide might be big enough to tell, I just don't have experience to know. I do have the narrower one on my bike that goes the furthest.