For horizontal dropouts I just pull it back as far as it can go and check the alignment of the wheel inside the chainstays and center it there. The alignment between the seatstays is usually not that adjustable with them.

I'd never thought to mount a wheel reversed to see if it's dished right, I usually sight along the side of the rim to the other side on both sides or try to pull a string along both sides of the rim from one side to the other to see where the center of the rim lies on the hub. That should tell you if the dishing is right. If its way off you can tell somewhat by looking. Normal wheels look very flat on the drive side.