Most employers don't test for Zoloft - it's not a controlled substance. Xanax (alprazolam), on the other hand, is commonly tested for in employment drug screens. It is a schedule IV controlled substance. It takes about 3 days to clear your system.
If you have a legal prescription, you're OK.
As for the posters who claim that employers don't look for "prescription drugs"... utterly false. The following controlled substances can all be legally prescribed:
Methamphetamine (given to KIDS for ADHD!)
Cocaine (administered topically for eye surgery, but can still be absorbed)
THC (active ingredient in marijuana)
... and lots of other fun stuff
Many employers who issue drug tests look for all or most of these, as well as others. The price for a 10-drug urine quick screen, when the tests are bought in bulk, is under $20. It's damn near as cheap to check for one drug as it is for 10. The confirmation GC-MS can cost over $200, but they only need to do that if the inital screening test is positive (more common than one might think).
If your script is up-to-date, you're fine.