Believe it or not, a dish gauge is very easy to make yourself. I used a nice stiff piece of plywood as the basis for mine. Making sure the edge was straight, I cut a notch for the hub and then used two small bolts and a small piece of metal to make a sliding indicator that goes against the lock nut. Then you just flip the wheel over to see if both sides meet the end of the indicator. Here's my awesome MS Paint rendition:
