Undoubtedly, there are a few readers who are wondering how one can tell how successful someone is with their tool handling and sharpening techniques by looking at the end results. As with any "rules", there are exceptions, but generally, there is a way to tell with some level of certainty. Sometimes the indications are obvious, and at other times, less so.
When you make a tool cut on a piece of wood, there is little to no variance according to grain direction and hardness of the surface wood. Because of this, the cut is very close to a perfect circle, and it matters not how refined that cut is. It may include some tear-out, but the geometrical shape is not effected by that. What is effected is the amount of sanding required to bring the surface (straight from the tool), to a surface that is ready to apply the finish. This is a key element, and exactly why we all strive to have the cleanest cut that we can possibly get with our tools, before we proceed to sand.
If it's understood that the resulting geometric shape left by the tool, and prior to sanding, is as close to a perfect geometric shape as will be had, then it's easy to conceive that sanding will alter that shape, according to sandability across long and end grain. The sandpaper will remove wood across the long grain more rapidly than it will across the end grain. As a necessary result of the sanding phase, the "perfect" geometrical shape will be altered. That alteration will be minimal when less sanding is required, and will be more, and proportional to the amount of sanding required. The more the tool finish requires sanding, the more change in the perfect circular geometrical shape will happen.......
Two intersecting planes result in a line. Now visualize two surfaces at a circumference point on a turning that intersect. Those two surfaces will result in a corner that will be a perfect circle along that intersection throughout the circumference. That would be true in a perfect world, but the amount of sanding necessary will alter the union of those two surfaces.......hence, the necessity of having the most perfect cut, straight from the tool, as the skill level of the turner will provide. Since this altered union between two surfaces will be visually noticeable, and have the appearance of being less than perfection to the viewer.......most turners will simply round over the corner so it's less noticeable.......this rounding over IS the indicator of tool handling, and sharpening skills, and there is no way to hide less than optimum initial surface preparation, other than rounding over the corner.
Another way to make an evaluation of the surface quality prior to sanding, is to observe that a turner doesn't use any detail grooves in their work.......or, when they do, to see how well those detail grooves appear to have been executed. If the surface under those detail grooves is as perfect as possible, then the detail grooves will look more expertly applied......duh! When I apply detail grooves, I try for less than .010" variation at that point, around the circumference. I use a machinist's dial indicator to determine this. There is ALWAYS some amount of variation, and the amount of that variation determines how many detail grooves can be successfully done, and the depth and width of those grooves. If excessive sanding is required for surface preparation, then it's impossible to maintain this .010" standard. This is exactly why many turners don't ever use detail grooves. If they did, the grooves would appear poorly executed......it is the plain and simple reason why they don't. (Making the cleanest cut possible while applying detail grooves, is a whole different subject, It is also a necessary ingredient to executing the perfect detail groove.)
These are the standards I use to evaluate some aspects of a turning's appeal, and are indicators of tool techniques and level of "sharp", when I observe turnings I can only see in photos.....and, they apply very effectively for that purpose.
ooc