Have a look through Bluebook. I got back about as far as 2010/11. We might have got a slightly different spec in NZ. You might be able to find a fork year stamp or serial number which will be within a year or two.
Edit Bikepedia is another good one. The only thing that's tricky is that Giant made a lot of Anthems in different specs.
Edit Bikepedia is another good one. The only thing that's tricky is that Giant made a lot of Anthems in different specs.