My collection spans 12 systems ( GameBoy, GameGear, NES, SNES, N64, GameCube, Wii, WiiU, Mega Drive, Dreamcast, PS1, PS2) and i got a grand total of 393 games for them.
At times, i find it hard, to keep track if i got a game for a system i play less (PlayStations being the prime example here, as these are mostly my GF's consoles) if i stumble upon something on a flea market for example.
So, i made myself an Excel-list (LibreOffice, to be more precise) and added each system as a separate sheet.
In each sheet, i have 7 columns (Title, Region, Disc/Module Yes/No, Manual Yes/No, Case Yes/No, Manufacturer code, Limited/Special/Silver Edition) and i think i could add a column for Spinecards/OBIs.
Since i only got one really packed display cabinet, i can't just go over there and have an instant overview - much less at a flea market

Also, i don't record any hardware i got - i recently counted my DC-controllers and VMUs and _now_ know that i got 7 of each

So, how do you keep track of your collection, what info do you write down?