I use a stereo microscope with up to 80x zoom to review the plates and parts before assembly. This is useful for spotting worn pivots/jewels/bushes/posts or parts that are not clean. As the parts are so small, it’s not possible to see issues without the microscope. Each of the parts is reviewed starting with the main plate and the bridges. Here are the train jewels in the plate:
And a close up of one jewel.
This shows the posts on which the dial side wheels will run.
Then the parts that run in these jewels and bearings are reviewed. Here is the barrel arbor and we’re looking for wear to the bearing surfaces in particular. These are smooth and fine.
Then each of the wheels (gears) are reviewed – to look for teeth damage. This is the second (centre) wheel.
And the pivots, again fine in this case.
This continues with all wheels through to the pallet fork.
And then the dial side components.
For this watch, all parts are fine but damage and wear is not unusual and parts are polished or replaced as necessary – this is essential for good performance and reliable running. Here is an example of a worn pivot where the right hand pivot may be clearly seen to not be parallel and is scored.
And here, an example of worn teeth on a winding pinion – compare the new pinion on the left to the worn one on the right.