Check A Checkbox If Not Already Checked And Vice Versa

I am trying to create a medical records inventory. We have physical charts which each have a barcode and I have a scanner. The idea is to have employees who want to check the chart out scan their barcode (on their badge), scan the chart's barcode and leave.

I have a table called tblCharts with the fields: ID (primarykey which is also the chart number), a checkbox, Employee ID, Date, and Time checked out.

I have Employee ID linked to my tblEmployees that simply has a listing of the employee ID's and first and last names.
I don't care about keeping a record of who has had the chart in the past.

I just can't figure out how to tell the checkbox to be checked (to indicate the chart is out) when a number is added to the ID field (via scanner).

So here's how it would go:

Scan the chart's barcode which would populate into the ID field. Search to see if the record with the same number is already existing, if not create a record. IF it does exist, check to see if the checkbox is checked. If it is checked then the chart is being checked back in (so uncheck the box). If it is not checked to begin with (still in), then check it out and associate the employee's scanned barcode with it.