You should be able to determine if it's plugged-in or not by monitoring the usb_device_state variable.
Code:
usb_device_state   VAR BYTE EXT
CONFIGURED_STATE   CON EXT

IF (usb_device_state = CONFIGURED_STATE) THEN
    ; OK to send
ELSE
    ; Detached or Enumerating, don't try to send.
ENDIF