Several types of probes are available:
Most people who have chosen to add a bed levelling probe to their printer have used an IR probe. This probe typically replaces the connection to the Z-min endstop.
Please Note: The ceramic coating on the original bed has a pattern that can cause inaccurate measurements when using an IR / optical probe. If you are using the original coated glass with an IR or Optical probe, it is suggested that you turn the glass plate upside down, and then add some black mat material underneath the glass to solve the issue.
Steve Wagg has put together an EXCELLENT guide on performing this mod on thingiverse. Steve has a modified version of the community firmware that is ready to go, configured for use with the IR sensor plugged into the Z-min socket.
You can test your ends top function using Pronterface, and entering gcode M119. This will report the status of all the endstops. You can put your finger under the sensor to trigger it, and then run M119 to see if the change in status is reported.
Photo credit: Marcus Harrysson on Facebook
Photo credit: Steve Wagg on Facebook