data checking

I am using a simple form for the user to enter data into the main table. one
of the fields available is "Box Number" and each of these boxes holds 16
items. i use a separate field "slot number" to designate where each item is
stored in the box. is there a way to store the data when entering it into
the form, so that each slot number can only be used on time for each box
number?

i want to create some sort of data checker or error message that will tell
the user "this slot number in box X is already used".

any ideas how i can set this up? cheers