Forms :: Compare A Text Field To A Table

I have a form that users put in information and then it updates to a table. It works fine but i would like to add something. Right now there is a text box called item that the user puts in the item number. The problem is that sometimes they put in a wrong number. What i would like to do is have it when they hit the update button to compare that item number to a table that lists all of them. so if the text box doesnt equal one of the numbers in the table dbo_item and the field itm_num, i want it to give them a message box with an error.