Make up your mind, do you want to talk about what is ervah, or what looks like ervah? Just don’t mix and match.
As far as knowing she’s single, some people do and some don’t. Same with married women. It shouldn’t make a difference according to your standard of “looking like ervah”, because it looks the same either way.
As far as a human hair shaitel looking like real hair, someone complained to the Steipler about that, and he replied “How do you know, you shouldn’t be looking”. Anyhow, the women claim they can tell what’s real and what’s not.
In this week’s Mishpacha, Rabbi Grylak related that someone complained to the Chazon Ish that a certain avreich’s wife didn’t cover her hair. The Chazon Ish wasn’t in the habit of giving direct mussar, so the next time he saw this avreich’s, he mentioned in passing that there is a store in Tel Aviv which sells real looking shaitel. The avreich responded that that’s where his wife got hers…