Shimano Nexus 8 Hub Alignment Mark No Longer Visible
You can flip the bike over; there is a second set of alignment marks on the bottom of the cassette joint.

Shimano Nexus Hub Adjustment



Scroll down to "Gear Adjustment"; there are drawings that are helpful. This page in general has great information on Shimano's internally geared hubs; worth the read.

You can also purchase a new cassette joint. I've never seen the adjustment marks become completely eradicated; if you cleaned the bike with strong solvents or cleaners, this may have lightened them.