How would you determine the quality of polish on the center stone of a lab-made diamond ring? For one, it should not misdirect light in an obvious way, or look dim absent any inclusions that could cause the same effect. Gemologists assign polish grades to each stone they grade, and this gets added to the official grading report accompanying it.
If there are no polish defects that can be seen with the naked eye, the stone gets an Excellent polish grade. Very Good means any defects are still hard to spot without 10x magnification and Good means pretty much the same. When buying, do not go lower than that.
Remember that Fair and Poor polish grades are both easily identifiable as having flaws leftover from polishing. When getting a jewelry diamond of that sort, at least make sure that it comes at a proportionally low price.