Identifying Arduino-compatible board signatures in code
In the boards.txt file each board has a board type definition, such as:

uno.build.board=AVR_UNO


When you compile your sketch that is adapted into a "board" define by prepending -DARDUINO_ to it and adding it to the compilation command line - so that one becomes the same as #define ARDUINO_AVR_UNO.  You can, of course, test for it with:

#ifdef ARDUINO_AVR_UNO
....
#endif


The location of the boards.txt file varies with different versions of the IDE, but every installed core will have one.  Look around in your Arduino program folder in the hardware subdirectory.