As others have said, you can generally feel the differences in diameter with your fingers. If you are so inclined, you can get digital calipers at Harbor Freight, if one is nearby, for a reasonable amount.
Re-entrant strings are always 1st, 4th, 2nd, 3rd in diameter from thinnest to thickest.
However, measuring string diameter might induce rabbit hole syndrome, so be forewarned.
John