Search For a Item on a container

My problem is simple, I think, but I haven't had much time to think about how to do it. I'm creating an item that functions as ProjectE's Eternal Density Gem but with sticks (i added like Compressed Sticks, that are 9 sticks). The problem is, I don't know how to detect when a player opens a chest or how to detect ALL the slots inside a container and see if the item is there.