How does one easily determine what the barrel inside diameter is other than using tools. Are these figures printed somewhere? How about bullet diameters?
Without tools, that is very hard to do. A bore gauge works well for this also. The inside diameter of barrels can vary from manufacturer to manufacturer, model to model, and even rifle to rife. There is no set standard that says all these barrels will be .501 diameter. That is one of the reasons some rifles, even the same make, will shoot one sabot bullet combination fine but a different one will hardly get it down the bore. Makes it kind of handy for bullet sales doesn't it.
As for the diameter of bullets, normally they are listed by the manufacturer. If your rifle has a hard time loading sabots, then you are better to look at the .429 or .451 diameter bullets for the .50 caliber.