From the NannyMUD documentation

LAST CHANGE

2001-09-29

FUNCTION

NAME
        is_player - Is the object a player?


LOCATION
        This function is located in /obj/player.

AVAILABILITY
        This function can be called by any object.

SYNTAX
        int is_player()

DESCRIPTION
        This function return 1, no matter the state of the player
	object. It can be used to detect that an object is a player
	object, with a single call_other, instead of having to take
	LD etc into account.


NOTE
	Do NOT define this in any object of yours. if you do, you will break so
	much code that it will be considered sabotage.