It'll be obvious when you install them. One of my 10sp Sunrace cassettes needed a 1mm spacer on a 10sp hub, and the other Sunrace cassette didn't. My Sram 10sp cassettes don't need spacers on my 10sp hubs but the Shimano ones do (1mm).
I have a number of bikes and wheelsets (trying to keep everything 10sp for now) and I find what works for me is to keep a stock of 1mm and 1.8mm spacers in the parts bin just in case.