Excellent article on alternative methods of checking progress from Birth Without Fear.  This article is up to date and clearly explains the red line up the bum crack, as well as many other ways of guestimating where a mother might be in labor, all without a vaginal check.  Super useful for laboring moms!

From Gloria Lemay, Pushing for Primips (1st time mothers)  (excellent info about vaginal checks, what signs to look for in labor, gentle 2nd stage)
How dilated am I?  from sarahvine

Using the finger breadth method (externally)

Dilation signs and how to check your own cervix.

More checking cervix info

External signs that indicate cervical dilation