When a disk system is present, main RAM slot ID can be read from following addresses:

• F341h = Slot ID of Main-RAM on page 0000h~3FFFh
• F342h = Slot ID of Main-RAM on page 4000h~7FFFh
• F343h = Slot ID of Main-RAM on page 8000h~BFFFh
• F344h = Slot ID of Main-RAM on page C000h~FFFFh

See also here if you develop a software in cartridge.