There is a macro specifically for the MKR 1000 board: ARDUINO_SAMD_MKR1000.
There is also one for the samd architecture used by the "Arduino SAMD Boards" platform of the MKR1000: ARDUINO_ARCH_SAMD, which will be defined if you are compiling for any board of that architecture.
There are some additional macros that were defined at a lower level by the toolchain. I don't remember their exact names, but I can likely dig it up if you need it.