As has been said you can use the topslide handwheel or a DRO if you have one.
Lets say the photo is a 0.221" dia rod. I would touch my tool against the lefthand shoulder and set the handwheel dial to 21. Then turn the smaller diameter stopping when the handwheel was on zero and just over two full turns away.
Alternative is to hold the rod or block against the left hand shoulder and bring the tool up to touch the rod where you would then set dial to zero, you then turn the smaller dia stopping at zero. You can also use a finished part as the gauge if you want one to slip onto the spigot and both ends flush.
Both these methods would be better than comparing by eye or feel.
I don't have gauge blocks so my suggested sizes may not be what comes in a set.
Edited By JasonB on 05/09/2019 13:22:42