One of the coolest things, in my opinion, is amniotic ferning. Amniotic fluid, when dried, creates a fern leaf like pattern that can be seen microscopically. The presence of ferns are a strong indicator that the bag of waters has broken. Two others are looked for: pooling of water in the vagina seen on speculum exam or visualizing fluid coming from the opening of the cervix and a positive nitrizine test (pH paper turns blue).
Its incredible the number of women who come in saying their water broke when in fact they either peed or have a lot of vaginal discharge. Both are pretty common late in pregnancy. I often wonder how many have a "high leak" for days before realizing that perhaps their water broke. I'd imagine that is more common than we realize.