I have just acquired a Type II #523768. The mechanism seems to work well, but how smooth is smooth? I am trying to figure out if it is worth getting it serviced. I have never handled one before, so I have no reference.

With all 0s on the input slides, the crank rotates very smoothly and easily, with a slight whirring noise and very positive stop at the end. Adding small numbers, I can feel something, but no noticeable additional force is required. Subtracting 1 from 0, so that everything is working flat out to make a load of 9s does require considerably more force, but is still easy and . I can do that using the tip of my extended little finger with no difficulty.

Other than a best quality Curta, is there anything I can measure against ?

Your description sounds smooth

It seems that your 48 year old Curta is running very well...
Once you will have a second or third Curta you will note slight differences.
In general older Curtas are less smoother than the newer ones.

Thanks for your advice. I shall not be sending if for service, not needed at present.

I figured a way, though a risky one, to assess the mechanism How fast can I turn the crank ?

Adding 1 (to both result and counter), four turns per second. Subtracting one (from both), three turns per second. I could probably go faster, but don't intend to try. If something does jam, that sort of speed will do serious harm.