The login detector has been a mod since SD stripped it out of the v0.5 release which was written for inclusion to threads 7.5.8; it will be stock in 7.6.0, the mod is available: [7.5.7+] Stop Forum Spam - Login Detection
A search option to see if users are in the db however does not exist, it's on the idea board at dev now for discussion at a later date. The only way currently to check if users exist is to edit their profile.
