In ACL 3 Low, you are seeing if the person can use their hands to grasp and release objects.
To Verify
Can the person sustain their attention to grasp, release and with different objects and motions? If the person can consistently grasp and release different objects this verifies ACL 3 Low.
To Rule Out
To rule out ACL 3 High, you simply need to see if the concept of a line captures their attention strongly enough so that they can color within a line and up to the line.
The concept of a line is strong when the person is functioning in ACL 3 High. This guides their actions so tiles are placed around the border of the copying activity. In ACL 3 Low, the border does not guide the person's behavior and they will place a few tiles down the middle of the frame or even up to a row or two.