In Design view, right-click the combo and select the Properties.
Go to the All tab, and scroll down to the series of rows that begins with Row Source.
First, check the Column Count. It is most likely 2 or more.
The Bound Column (this is the data that Access stores) will be 1, more often than not.
Now lok at the Row Source. Click the Builder (...) button in the Row Source line, to bring up the query grid. If Bound Column is 1, the first field in the grid is the one that you have to filter by. If it is 2, pick the second field, and so on. Close the grid but don't save.
To confirm which field the value is stored in, go to the top of the property list and check the Control Source.
Denis