Well, I have a simple parallel port chinese TB6560 controller and I use one of the input pins on the controller setup in LinuxCNC as a probe. There are 5 input pins on my controller and 1 common, so with a lead connected to the common and another connected to the input.

With the probe attached to the tool, the Z axis comes down and when the tool makes contact with the board, it measures the height and adjusts the rest of the g-code to suit the board at that point.