Form text box entry that looks for duplicate

[Daiuy]

I'm trying to create an application that controls visitor's badges to our
facility.
On frm.AccessControl I have text boxes where I enter the visitor's name,
organization, address, the arrival and departure dates and the badge number,
badge issued date, and badge returned date.
I want to ensure that one badge can not be issued to more than one person at
a time.
What I'd like to happen is when I type in the badge number into the
BadgeNumber textbox it will search the table for this specific badge number
and then check the badge returned date field. If the badge return date is
null, I'll get a message telling me that badge is already in use. If the
badge return date field is not null, no message and I can continue.
I have very limited experience, so please keep it simple.
Daiuy