How can I tell if the spy I just killed is using a Dead Ringer?
Is there even a way to tell if they feigned their death (without additional Spy-Checking)?
I'm not sure I should be giving away my tricks, but...
Not all on hit/on kill effects are replicated by the Dead Ringer. Some things that could give it away are:
Things that make seem like a Dead Ringer activated them, but may not be:
Take a look on this table in the row "unreplicated mechanics" that shows effects that don't apply as they would normally do.
Besides this, there are some classical signs the spy is still alive:
All the listed answers above are correct, but I have one more:
You can see the spy's cloaked outline for a split second as his rag doll counterpart falls to the ground. Try looking for that.
Whenever you kill a spy, use the "thanks" voice command. If your character says a generic "thanks" (not referring to a specific person) then the spy is still alive. If they say something like "we make good team" or "I didn't need your help, you know" (referring to a specific person) then the spy is dead. I'd say this works about 95% of the time. Only time it won't work is when the kill happens under extreme circumstances, it's not sheer luck.
If you want to get used to using this just say "thanks" after every kill. This will get you used to the voice clips used when you get an actual kill.
I have yet to see this mentioned anywhere else. Am I blind, or did I actually discover something?