The easy way, beside the ways Dave S, Denny and Tech said is that the sharp edge always goes towards the liquid being held back no matter where.

And most likely you will need the sleeve as Denney said as there is probably a groove worn on the damper shaft. They are only about $7-8. You will probably have to dress the seal surface down before you try to install the sleeve. Use some fine wet 'n' dry (220 - 320 grit) sand paper.