Yeah man - it is not unusual to test for benzos.
To generalize, unless you are getting a very high clearance from the NSA, Military, CIA, or some State entity your test will probably be an immunoassay (sp?). They will test for some predfined list of drugs and if any of them exceed a specific cutoff level gthey will then do a gas chromatograph test to nail down the exact level. In the case of benzos, becasue the first test will just identify benzos and not the specific type, the second test will be used to identify the specific compound. Then you will have to provide evidence that you are legally taking that specific benzo and probalby explain why you forgot to mention it in the first place.
Xanax has a relatively short half life so stop taking it until the test and maybe it won't exceed the cutoff level.