Signs that large rockfalls may be about to occur include -
widening cracks, IE, topo says 2" and crack is 6"
small rockfalls from the base of flakes and pillars
creaking, groaning noises
grating sounds
dust drifting out of cracks
new cracks forming on what used to be blank faces or clean slabs
if you see any of these signs, run away
some or all of these were reported immediately before the rockfall on Trango tower (see Alpinist 11) and the 1990's rockfall on the Dru in Chamonix.
neither of the above mentioned rockfalls had anything to do with rainfall.