I suppose it is possible that a temporary contractor has an email address from that company, but I think you can get reasonably high signal by looking at their LinkedIn history. Switching positions every few months is a red flag. Even if they're an independent contractor, longer-term arrangements with each client suggest better relationships and better commitment to real outcomes.

I see plenty of recruiters who just work for recruiting firms and don't hide that fact. Anyone who won't immediately tell me who they're recruiting for gets ignored. I'm sure I end up talking to the occasional contractor but you can easily filter out a lot of obvious low-hanging (and rotten) fruit).