if(GetThingType(potential) == 10) // then thing is a player
With SetThingType(), it is possible to change a thing's type ingame. But this is not recommended.
The first two thing types listed here, free and camera, are not known to function and were never used. But they are in an exe listing of thing types. And this listing is in order of their thing type number.
Note that these thing types are not related to message types. Message type values are a property of messages and are not used with things directly.