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From what I have seen, the SK7 (which I own a couple of), has a gray bezel and the SK8s that I have seen in person (fellow club members) and shops have a more silvery looking bezel.
If you take the compass capsule out from the protective boot so you can see the underside of the card, you should be able to see the magnetic chip on one side (north/south) of the floating card in the SK8...I have pulled my SK7 capsule to check the NH/SH model orientation and did not see a chip...so I assume no chip = SK7, chip = SK8.
If you bought yours new the packaging would indicate which you bought. If you bought yours used....there is not much of a difference to be all that concerned about unless you were advertising it for sale and want to know for the product listing.
Both the SK7 and SK8 seem to suffer the same fatality of developing hairline cracks in the face of the capsule which causes the oil to leak out and an air bubble to form. The bubble eventually becomes big enough that it jams up the rotating card at depth making the compass useless.
Depth and pressure gauges with lexan/plastic capsules seem to be indestructible compared to the craptastic plastic that Suunto is using for their compass capsules.