That and seeing a bunch of it. If you look really close at untreated the surface looks rough and the edges fray, DWR the edges fray and it is shinney, sil the edges do not fray and it is shinney.
Make sure you look for the box pattern if you want ripstop. I have been seeing a decent ammoun of non-ripstop or taffeta. I am a little fuzzy on the differences.
If you want www.owfinc.com
sells fabric samples for cheap that would be a good reference.