G-code (M-code?) for Get Bed Temperature
M105should give you the bed temperature.
For future reference you can find a general list of G/M codes here - RepRap Wiki - G-code.
Most firmware files include a list, Marlin has it in Marlin_main. I have no idea how often the list is updated but they don't change often.
M105 is the extruder temperature. I can''t find a way to get the bed temperature, though there is a code to wait until the bed has reached the target temperature.