Measurement of a signal at microcontroller's output pin
Does the control board have JTAG? JTAG offers "boundary scan", a way of determining each and every input and output pin on the chip - plus the values of internal registers and all sorts of other things. Of course, it means you need it connected to the JTAG chain, and a JTAG connector, and a JTAG probe, and software to drive it.

Otherwise, @ScottSeidman is right: you're out of luck.