Its good practice to include either G20(Inch), or G21(Metric) in a safety block at the start of all programs. Although conversion of offsets from metric to inch (and visa versa) is dependent on parameter setting, having the correct Unit Mode for a given program via the first line safety block will circumvent a monumental crash if a Metric Program is fed to a control that is in Inch Mode (Z-50.0 in a metric program will equate to Z-50.0 inches if executed with the control set to G20 (Imperial Units).
The control can be changed from one Unit Mode to the other by doing as Jashley73 suggests and executing the appropriate G code (G20 - G21) via MDI, or by including the correct G code in the first Block of the program. G20 - G21 are Modal within the program and will also persist as a default when the power to the control is cycled.
As Jashley eluded to, there's a 'settings' page. You may have to press your Offsets button a couple of times to get there. After you've pressed it a couple of times you should see a page containing the setting (either 1 or 0) for inch/metric. It works on our 18i and 21i controllers. I'm not sure if that's what you're looking for or not and I've never fooled with it myself to see the results.
We have a very old (1984) Fanuc 3T that you have to press a key that says "Param" to get to it.