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Supposedly you can sometimes tell by doing a burn test. I doubt if it's fool proof though. I haven't tried this so do be careful if you do.
I usually determine if something is wool by trying to do a felted join of yarns. I take two pieces 5 inches long or so and wet one end of each then roll and agitate them in my hands to see if they will felt together..same method you use for joining in a project with wool. Superwash wool won't do this though so it's only good for feltable wool.
As for what you can mix together.. as long as it's about the same weight/gauge anything that looks good can go together.