Look at the trucks. Are they roller bearing trucks? If not, 33. If they are roller bearing trucks, look at the weights stenciled on the side of the car.
For cars with 2 trucks, take the LT WT, and the LD LMT and add them together.
If the number is...
Under 220,000 - 33" wheels
Under 286,000 - 36" wheels
Under 315,000 - 38" wheels
Above 315,000 - You'd have to check for coupler height to determine as it would most likely ride on more than 2 trucks
There are exceptions. For example, most 50' box cars regardless of weight/load limit will use 33" wheels. Bi-level autoracks will use 33" wheels while tri-level autoracks will use 28" wheels despite being heavier for vertical clearance purposes. Front Runner flat cars will use 28" wheels due to the light weight of the car.
Most manufacturers will tool their cars to be dimensionally correct from the bolster up. Below the bolster is another story because HO scale trucks are not mounted the same way as real ones, and the height of the bar (not a technical term) that the bolster sits on, relative to the sideframes, is variable on the prototype. So, if the height is too low, make sure you have the right size wheels. If you are sure that you have the right size wheels, or if the right size wheels won't fit because it will catch on the underbody, and the height is still too low, shim the car up. As I've said before, the height from the bolster to the roof of the car is a set measurement if the manufacturer tooled them to the true-to-scale dimensions. If you use an offset coupler to raise a coupler on that car, the car will ride too low, and won't look very good next to other cars that have the correct height.
I've only had to use the overset (height lowering) head coupler once. It was on an Atlas Trainman bulkhead flat. I have two; one from the first run, and one from the second run. The one from the second run rides at the correct height, but the couplers are too high. To this day, I still can't figure out what is causing it.
