[ARCHIVE ANSWER]
Yes. They can tell the difference with a test if they look for the actual compound itself,
but they can fire or not hire you anyway if you test positive for synthetic THC because
they don’t normally use that test. Such a test is both sophisticated and expensive, so it has
only been used for a handful of high-profile federal criminal cases.
When they perform a normal drug test, often times they are looking for metabolites of
THC, not the actual drug itself, which means that the metabolites from Marinol are
exactly the same as THC. A lab wouldn't be able to tell the difference in a person who did
one or the other without looking at the actual compound.
The GCMS drug test was invested for use on the military and other public servants in the
United Kingdom; however, the insurance companies got wind of it, and it trickled into
the private sector. Parliament was going to pass drug testing in the UK, and the inventor
went back and told them about the accuracy and asked that they not do to their people
what the U.S. government has done to us. He made himself a billionaire at our expense.
In your case, you could make a big stink about your drug test and try to have the tests
thrown out. Marinol is an FDA-approved drug, so if you have a prescription or even a
recommendation from a doctor it, you might be able to bring your case to court, have the
results thrown out, and/or sue them for discrimination for firing you for it in such an
instance.