Not a mark seen that often, but I could pick out the unmarked white lesser coverts that creates a contrasting, though small, area at the base of the wing. This area is uniformly marked with the rest of the underwing on Short-billed.

For a look at Short-billed overhead: http://www.pbase.com/mctodd/image/112483054

Noticeable even on distant Long-billed: http://www.pbase.com/mctodd/image/111740947

This mark is easiest seen in photographs though I am sure...This is the only image posted of these shot with my 400mm and not digiscoped.