I do something similar to test the 'scope.
First I get the reticle roughly optically centred by supporting the tube in a homemade double vee block arrangement screwed to a tripod. You could make something more professional from e.g. a pair of high air rifle scope mounts.
Aim the scope at a suitable fixed object and rotate it in the blocks. By adjusting the turrets you will be able to get the crosshairs to be fixed on the target instead of rotating in a circle as the scope is rotated.
Then I clamp the scope in the blocks and check the turrets move the image smoothly and evenly and pass the box test, and have about the same range of travel either side of the optical centre.
Then I re-centre the scope and fit it. Usually its quite close to the boresight.
If the 'scope is fitted to a rifle I use an optical collimator mounted on a spud in the muzzle to check the turrets are working properly. Its surprising how often they are sticky or jerky or don't return repeatably.
This is the tool I use:
Opticswarehouse Boresighters
The boresighter is also handy for quickly checking that the zero has not shifted before using the rifle, for confidence. Make a note of which grid square the crosshairs are on. If it has altered much between outings it would be worth testing zero.
I think the boresighter has more than paid for itself just by revealing that one brand new scope was defective, so I didn't have to waste time and ammo discovering it.