Most reloading manuals will have a section that you can calculate bullet drop over range but you need to know several things.
Bullet coefficient.
Muzzle velocity.
This will get you in the ball park but,,, the others have the best idea. Shoot at the top of a target from a bench hold at 50 yards and get a decent pattern then move the target to 100 and use the same place on the target.
One thing, you must have the height of the gun above ground the same (close as possible) to the height of the aim point of the target. At 0 yards, 50 and 100 yards. Otherwise your adding or subtracting height to the bullets trajectory as it goes down range and making this kind of test useless.