coolant level? remove expansion cap and listen for glugging or watch the level drop,look for any leaks inside car.If you have antifreeze in your coolant you should be able to tell wether its coolant or rain water coming through as antifreeze feels a little oily and if you wipe it off your hand it should still feel oily and it tastes sweet (dont taste too much!!!!!!)
scuttle tray full of water?/dirt/leaves if you park near a tree this will block drain holes.
does your heater work?