You'll need to do super-precise measuring of the ball's diameter (in hundredths-of-an-inch) and weight (in pounds and ounces). When you've gotten those measurements, go to the following webpage and look for an exact matchup with the diameter and weight of a historical cannonball.
www.civilwarartillry.com/shottables.htm
I co-wrote a detailed instructional article on how to tell if a metal ball is an artillery ball (cannonball, grapeshot ball, canister-ammo ball) or not. The article includes helpful photos and instructions for how to do super-precise measuring. You can view it online for free at:
SolidShotEssentialsMod
There are literally multi-millions of civilian-use and industrial-use iron/steel balls, such as:
Mining Industry rock-crusher balls,
Sports Shot Put balls,
machinery counterweight balls,
Ornamental Ironwork balls
cement delivery truck tank cleaner balls,
ball-bearings (some are quite large).
The only way to tell if a ball is an artillery ball instead of a civilian ball is by super-precisely measuring its diameter and weight, then seeing if there's a matchup for it in the historical artillery ball size-&-weight data.
