For an end of the cylinder's axis, using a three point circle off either the appropriate midpoints of the edges of the facets that represent the cylinder walls, or the vertices of those facets. Not an infallible method, because it'll be off if you pick a mixture of both for the same circle, or if the surrounding end faces aren't normal to the axis. The edges should describe a regular polygon, but that's not guaranteed either.

*forgot to mention that you've got two diameters that way, too. The midpoints of the facet edges are more likely to produce the right diameter, but that's not a given, either....