I have three methods, which I will list in order of probable use:
1. Experience. One does develop a feel for the DoF with a particular lens/format and distance. At least enough to know when a more rigorous approach is advisable
2. Use tables or a DoF calculator. This depends on an accurate distance measurement, which the Mamiya TLR will not give you, to work well. On the other hand, people can learn to judge distances better than the precision with which you can set the camera.
3. There is a view camera technique that uses the difference in near and far focus points. To use it you need to be able to measure the lens extension change. A millimetre measure can be used to do this, or an inexpensive dial caliper. The range of motion for normal photographic distances (say 4' to infinity) is not much, so your error is likely 5-10%.
There used to be a device like a dial gauge that measured the extension from the accessory shoe to a plate clamped to the lensboard. It was directly calibrated for distance with the shorter lenses.