I always wondered about this. How do multi-slot mvs boards recognize when there is a cartridge in a slot? I suppose this applies to 1-slot boards also since it needs to know when to cross-hatch.
There's some detail on the the coherent data the BIOS expects on the wiki . Off the top of my head, it checks the "NEO-GEO" string @ $100, a non zero NGH value @ $108 and a valid security pointer @ $182. If any of those don't check out then it considers the slot empty.