How is the best way to see when your tabs are neutral. Not dragging but in line with hull. Do you use a srtaight edge while boat on trailer and mark the indicators?

Exactly. Use a LONG straight edge, i.e. 4+ feet to make sure you don't follow a hook or rocker in the hull...
... and use a tape measure to make sure the trailing edge and hinge edge at the transom, are equidistant from the straight edge.