How to know if player has an item in his bag?

[Mitsarugi]

I'm trying to make a function for TreasureMaps that let's me check if the player has the item (treasuremap) in his bag so that i can display that Treasuremap's location on the map.
but i need help as i don't know how to do this...
It would also be nice to know once the item gets consumed (upon finding the Treasure itt gets deleted from inventory )