How do I tell which box of contacts is for which eye?

If they both say D -3.00 you may wear the same in each eye. However, when ordering through 1800Contacts I don't think they usually label the boxes for you; you're just supposed to look at your prescription that you ordered with. Remember OD is Right Eye and OS is Left Eye. If you're still unsure, call your doctor's office and they'll be able to look in your chart and tell you.