An actual way to spot spoofers?
- The best way would be by checking the player's journal (also Pokemons, items and candies). However, you obviously need the permission of the player to do so, which is very unlikely to happen (spoofer or not).
- But what about using one of their tricks against them, breaking into their private lobbies:
https://www.reddit.com/r/TheSilphRoad/comments/7u9nce/how_spoofers_can_enter_private_raid_lobbies_and/
The idea is simple, it is very likely that in order to increase their chances to participate in ex-raids, a spoofer will solo low tier raids in eligible gyms (in their own private lobby, if willing to remain anonymous). If that is the case:
1. As soon as the raid egg hatches, create your own private lobby.
2. Quit and proceed as described in the link above.