Aside from the talcum, another good trick are washable children's markers. They're like magic markers--but water washable, sold with the kiddie art supplies. You take a big thick one and draw a big arc around the packing gland or "downhill" in front of it, and check back on the next day. If water has been trickling in, it will wash away part of the line.
Same idea as the talc, but if you are looking for one specific area and don't want to scatter talc, it's a bit neater and cleaner. Also easier to see than white-talc-vs-white-hull in tight spaces.