Can I get a wheel dished without the bike?
Yes, a dished wheel can be built or re-dished without a frame handy. 

In order to get the wheel to revolve, the rim must be equidistant between the insides of the stays/fork. The reason why most rear wheels/front wheels with disk brakes are dished is because the hub flanges aren't centered between the ends of the axle. This is to accommodate the width of a cassette on a rear wheel and the space taken up by a disk on a front wheel. 

To accomplish the desired dish, a mechanic will lace a hub to a rim using spokes that will result in the correct rim placement. There are all sorts of calculators out there to correct for the offset from the hub flange. The spokes on opposite sides of the wheel will be at different tensions and lengths, as opposed to a dishless wheel which has spokes at the same tension and length on both sides. People commonly break spokes on the drive side of their rear wheel - this is because those spokes are generally at a higher tension than those opposite it. Ideally, a wheel is dishless so as to evenly distribute the stress on all spokes. To achieve this, there are many offset rims which compensate for some of the hub flange offset. 

However, none of that will effect the disc or caliper function unless you've misplaced some shims reinstalling the disk or moved the caliper. If you have hydraulic brakes and pulled the brake lever without the wheel installed, the brake pads will probably just need to be pushed back into the caliper housing with something flat and blunt.