As I mentioned in a previous question, I'm in the process of replacing the stock rear wheel on my Giant Escape 2 (2016)(any comments/answers related to why and whether I need to replace it should be entered there). The one thing I can't seem to find in the specs is whether I have a 130mm or 135mm wide wheel/hub/or whatever that measurement pertains to. How do I find out? How do bike shops know? If I measure this myself, what do I actually measure--the rear wheel dropouts or something else?
The measurement there is known as "O.L.D." or Over-Locknut Diameter. It's the width of the opening in the frame between your dropouts, with a hub installed. So from outside-of-locknut to the opposing side outside-of-locknut on your hub. This can be measured with calipers or (more difficult but possible) with a ruler. Remember in this case you need to be very accurate in measuring, since it's either 130 or 135 millimeters, and that 5 millimeter difference is very small(about a quarter inch). If you're sure about the type of bike you have, this could be an easier way. 130mm hubs are generally only used on true road bikes(700x~20-25c). 135mm is used on Mountain or Hybrid bikes(any 26", 29", 27.5", or 700x~30-50c). Looking at Giant's website, this should be a Hybrid, and 135mm OLD.