George: Try this. it may give you a rough idea.
you will need some graph paper, a good pencil and a decent compass preferably from a drafting set, and a drawing triangle or small square.
remove drum and place down onto a sheet of 1/4-inch graph paper or similar.
Using a sharp #2 pencil trace the outline of the drum , that is the outside circumference onto the paper.
Go inside, pour a cool one and proceed. We will use some geometric construction to find the center of the circle which represents the brake drum.
see sketch below
Adjust the compass to a radius a little greater than what you have. In this case 6 inches should work.
At a point on the circle, say at 12 o'clock, we will cal this point 'A' place the pivot of the compass and draw an arc which intersects the circle at point 'B'.
Without changing the compass, place the pivot on point 'B', and draw an arc which intersects the circle at point 'C'.
Now, carefully draw a straight line and connect 'A' to 'B' and 'B' to C.
Measure line AB and BC. They should be equal or very close.
Find the center of these lines (AB and BC) and draw a perpendicular line from the center to a point beyond where you suspect the center of the circle should be located.
The point where these two perpendicular lines intersect is the center of the circle which represents your brake drum.
Mark the center point. Now with the pivot of the compass on the center, adjust it to touch the circumference at any point on the circle. Draw a circle using the compass. If the circle is true (if the drum is round), this circle should coincide with the outline of the brake drum.
The amount of disagreement is equal to the "out of round" of the brake drum.
Good luck
Frank