If unboxed how could you tell the difference between refurbished imac and new imac?

I've purchased several different refurb Macs and have never been able to tell the difference - on an iMac, there was a brand new keyboard and mouse; a Macbook looked like new, and a Mini did as well. The warranty is the same as for new products, so to me, it's always been a good choice.

Besides all of that, you can purchase Apple Care which will extend your warranty to 3 years just like you can on a brand new iMac.

If you're considering a "refurbished" iMac, buy it ONLY from Apple. I believe it's your best bet.

Actually, it's the only "bet" - refurbished (by Apple) Macs are only available from the refurb section of the online Apple store; anyone else (reseller or whomever) selling "refurbished" units will most likely have done the work themselves and it wouldn't be covered by an Apple warranty.