Yeah, check the serial on the case ( marked e.g Serial: xxxxxxx ) just under where the model # is, and then check on the underside of the box your PS2 came in and next to the barcode is a little sticker, with two bar codes on. The serial should repeat underneath the second barcode. It may look like e.g. S01-xxxxxxx-N
If the numbers don't match up, your PS2 has been swapped.
That's true for UK consoles anyway, I don't know about US...