Yeh the most accurate way to sex them is by the presence of nuptial pads on the "thumbs".
Males have them and females don't. I hear males that aren't in breeding condition have reduced pads and as such may be harder to see. Males also have looser throat skin than females(for calling). The general overall size of the frog can also be a good indicator. Male cranwelli grow to about 3-3 1/2 inches Females to about 4-4 1/2.