Francis Bollag suggested this DIY dishing gauge a while back. It consists of a flat table, 3 or 6 tuna fish cans, and a stack of pennies. Support the wheel by the rim at 3 points by the tuna fish cans. Two cans should be sufficient so that the axle is above the table surface. Use the stack of pennies as your measuring gauge to the locknut. Flip the wheel. Adjust the dish so that you are a penny width between the two sides.