What's the best way to find out if my identity was stolen?

I’ve been getting spam from an apparently real debt collection company but the thing is I’ve only ever owed debt to two entities: my credit card and the government (fucked up my withholdings once). I know I don’t owe shit, so either it’s a scam that’s impersonating a real agency or somebody stole my identity and they think I do. Which is possible, for reasons I won’t go into.