"The white spots along the edge of the hindwing above are larger in P. oileus than they are in P. comunnis. In the former, these marginal spots are almost as large as those in the inner, submarginal rows. This and the brownish underwings of oileus are listed by most authors as the distinguishing features between the two species. Few references, however, cite the best distinction, especially in the field. The tropical checkered skipper, P. oileus, has three dark blotches along the leading edge of the hindwing below; these marks are not present on the common checkered skipper."

BugGuide's images of the Common Checkered Skipper also have a the row of large white marks on the dorsal base of the hind wing. So that may be less distinctive than the mark on the dorsal forewing.

The Tvetens' description of the ventral sides seems to agree with what BugGuide has. I can make out the marks on the leading edge of the hindwing of the Tropical Checkered Skipper, that the Tvetens mention. And notice the hindwing of the Common Checkered Skipper has alternating bands of color from the abdomen towards the forewing (The Tvetens' "dark gray or olive-gray".). On the Tropical Checkered Skipper, the bands are almost all the same color.

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to the Common Checkered Skipper.

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to the Tropical Checkered Skipper.