How can I determine the current value of a #defined variable? I previously set up my Ender 3 with a precompiled binary of Marlin that already had the correct X and Y offsets for the attached BLTouch. Now I'm compiling my own, and rather than measure for myself I'd like to just use the same numbers.

When I send M851 through the terminal, it gives me the current value for the probe's Z offset from the extruder. I know I can also use M851 to set the X and Y offset, but there doesn't seem to be a way to read the current values.

Is there any way to query the printer for the current values of X_PROBE_OFFSET_FROM_EXTRUDER and Y_PROBE_OFFSET_FROM_EXTRUDER? Or, ideally, for any other arbitrary variable?
"#define" is a feature of the compiler preprocessor and isn't a feature of Marlin. So there is no general way to recover the value of a "#define variable" (which isn't actually a variable at all) unless Marlin explicitly provides the option.

In the current version of Marlin M851 will report the X and Y offsets. It appears you have an older version which does not do this. In older versions there appears to be no option to recover the value.