As I stated in you other question, pre-tax insurance lowers Box 1 and Box 3 and Box 5. So the W-2 does NOT tell if it is pre-tax or not. You need to look at your pay-stubs to determine if it is pre-tax or not (and the can sometimes be difficult to tell). As both Katrina and I said, it is quite unusual for health insurance to NOT be pre-tax.
Box 12, Code DD is for insurance through the employer. It does not matter if it is pre-tax or not.
