How can I determine the current value of a #defined 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.