Function GetPlayerWeapon returns the ID of the weapon a player is currently holding.
When the player state is PLAYER_STATE_DRIVER or PLAYER_STATE_PASSENGER this function returns the weapon held by the player before they entered the vehicle. If a cheat is used to spawn a weapon inside a vehicle, this function will not report it.
Sometimes the result can be -1 which is an invalid weapon ID. Circumstances of this are not yet known, but it is best to discard information when returned weapon is -1.

int playerid The ID of the player to get the currently held weapon of.

Return Values:
The ID of the player's current weapon. Returns -1 if the player specified does not exist.

public OnPlayerDeath(playerid, killerid, reason)
    if(killerid != INVALID_PLAYER_ID)
        if(GetPlayerWeapon(killerid) == 38) Ban(killerid); //Ban if they have a minigun
    return 1;

Related Functions
The following functions may be useful, as they are related to this function in one way or another.