there are 5 ways to know if a spy used dead ringer (that im aware)

1. the ammo box. after killing a any class, they will drop the great, the famous ammo box (yes even with dead ringer), but here's the catch: if you walk over the ammo box and you notice you don't get any ammo that means the spy used dead ringer

2. the player tab. this one is easy, if you "kill" the spy using the dead ringer top right it will say that you killed him, BUT if you press tab and search for the spy's name and its not kinda grey that means he's alive, alternately, a easier way is if you search on the top bar with all the players, which will show if an enemy is dead and what class he is (if he's dead), you do the same with the tab crap and if you see no new people dead that means the spy is still alive

3. the loud ass sound. use your ears, if you hear the loudest sound you ever heard of a decloack that means the spy is near

4. keep holding LMB. also easy, if you have an automatic weapon and you think the spy used the dead ringer, just hold lmb and search around the zone, if the spy didn't die you will probably hit him/burn him

5. the spy being an idiot. if the spy walks into you and bumps you, you will see the spy become semi-transparent, now take your chance and beat the living daylight outta that ♥♥♥♥♥

sorry if there are grammar mistakes my main language is not english ok i hope you enjoyed this guide now go kill the spy mains