If the patient starts maniacally laughing, then that is a sign of amogus.
Serious victims typically produce more uncontrollable and potent laughter and more objects are able to trigger it.
Text chat is not a great vehicle for psychological subtlety.
Impostors likewise can be Shapeshifters, allowing them to temporarily morph into other players.
Innersloth also stated that the map would be free to all players.
Note, if you have a huge number of players, you can also select to have multiple imposters.