Also the float may have sunk. As for the needle and seat, you can usually look at them and tell if they're worn out. As for finding it take the float bowl off and you'll see the float dangling there. The needle is what the float is attached to and moves up and down with the float into the body of the carb, seating against the seat.
To check the float for floatability, if it's brass, shake it and listen for gas sloshing around inside. If it kinda plasticish, some are hollow and you can shake them, but some are foam and you need to squeeze them lightly and see if gas squeezes out.
As for missing parts, that's pretty unlikely because the only parts that control fuel going into the tank are the float and needle. I can't think of any ball bearings in a carb! Do they make the fuel flow faster? Even with the gas on at the tank , the fuel shouldn't be running out without some problem with the needle and seat.
Bare