Detectives are ALWAYS innocent (it's a specialized role).
If the game isn't over and there are only 2 people alive, then the other person must be eliminated,since you know if you are innocent or not.
If you see someone with a Traitor Weapon , they are probably a Traitor, since such weapons can only be bought by traitors, although Innocents can pick them up from dead traitors.
Detectives have DNA scanners which you can pick up if he/she dies. If you use it on a Corpse it will show a marker where the person who killed them was since the last scan, unless they have planted a Decoy.
Decoy - Can be used on a body to falsify evidence, causing the location revealed by a DNA test to be incorrect.