How was the horsepower of farm steam engines determined?

Horsepower of farm steam engines was greatly underrated. Most manufacturers gave a nominal rating.

Many people refer to this rating as the "drawbar horsepower," implying the power the engine has in pulling something.

Other companies gave a belt horsepower rating (always higher than the nominal rating), implying the power the engine has at the flywheel.

The J. I. Case Company came to rate engines by belt horsepower.  A 60-horsepower Case is nominally a 20-horsepower engine.

Some firms gave both ratings. The Frick Company dispensed with horsepower ratings and advertised engines by the size of their cylinders.

60-horsepower Case in 1919 catalog	9 x 10 inch Frick in 1912 catalog
20-70 Nichols & Shepard in 1914 catalog	20-horsepower Garr-Scott in Power Farming Machinery