Newbie needing help part 2

well i got another problem now.. i know how to give a NPC a item but how do u make the NPC know if u have or not for example:

if i had a guard blocking a gate and he would only let me pass if i gave him the key but when i tried this he let me pass but i didnt have the item on me...how do i fix this?

if u can help me it would be great