%player.getMountedObject(%slot);
Note that whatever comes before "getMountedObject()" should be a player of some sort. So you could also do
%client.player.getMountedObject(%slot);
FindClientByName(blockhead).player.getMountedObject(%slot);
also, doing a function on a player simply sets the first variable in the function as the player. So, really, you could do
getMountedObject(%player, %slot);
but the most common method is the first kind.
EDIT:
By the way, %slot is what body part to check.
For example, 0 is your left hand which pretty much every item in the game is held in.
Play around with it to figure out the rest.