Park Tool Portable Wheel Dishing Gauge - WAG-5 $32.95
The Park Tool WAG-5 Dish Tool accurately checks the dish, or offset, on all 26", 27", and 700C wheels. Measurements are taken comparing one side of the wheel to the other to determine if the rim is in the center of the hub. The Wag-5 easily folds in half and fits into most tool boxes. It's perfect for the race or home mechanic.
What community has to say
Yes it is a necessary tool.
I have seen a few people try and true their wheels and totally pull them out of dish.
I like my bikes to be perfect and this is a necessary tool if you want to build your own wheels and especially if you own your own truing stand.
I remember walking through the local REI getting little travel accesories before we headed for Honduras to teach DH and XC skills for two weeks and hearing one of the costumers complaining to one of their techs about how his tire was lining up between his fork legs. The idiot mechanic there was bullshitting him about how the fact his front hub had a brake rotor and it is what throws the wheel out of dish when you put the wheel and tire back into the fork. Yeah, I butted in and called bullshit all over the guy. Doing your own bike work is really the only way to avoid some idiot fucking up your bike.