While calipers are incredibly handy, and while cheap junky calipers are astoundingly good (this coming from someone who has more in his metrology gear than you do in your bike), you can tell those two diameters apart with a grade school ruler. Or by checking it against a know item, or by seeing if it’s bigger or smaller than your known size seatpost.
They really don’t make pennies like they used to, at least not like before 1982. That said, if you’re failing a penny you’re probably trying to spread the stem more than you should. Better the penny as a fuse than part of your stem.
So long as a stem has the appropriate length and diameters at the ends along with a reasonable angle, the rest is a matter of non-critical preference.