At most big festivals there is free drug testing by harm reduction charities. Not that they'll test you for drugs, but they'll test your drugs to make sure they are what you think they are.

At Fusion Festival, I saw a big bulletin board completely covered in notices of "we tested this pill, here's a photo, here's what they thought was in it, here's what was actually in it"

They also spelled the name of the charity wrong on all the maps, so that's nice.

I think the drug users will know before the Opus users.