i just went thru this situation. actually they can tell from your serial number. if purchased from at&t, it's one year from purchase date. if bought from another vendor, like e-bay, it is 15 months from manufactured date. i had a htc touch dual that i bought off of e-bay, was sold with 12 month warranty, so i thought. when i called htc to do a warranty claim, i was informed that it was not still under warranty. lucky for me the seller took the return.
hope that helps.