Chris, unless you know what to look for, a visual may not help much. There is an easy test you can do. Close all windows and exterior doors, fireplace flues, etc. and open interior doors. Crack your front door an inch or so, then turn on the air furnace air handler and note whether and which way air moves through the door.
Your basement doesn't have to have standing water to supply a good deal of moisture. You can tape down a sq. yd of poly for a couple days to see what collects under it, if it's bone dry look for other sources of moiture.
Anything that alters the pressures in the house should be looked at, so your whole house fan is worth considering. This should depressurize the house and pressurize the attic. The degree to which the attic is pressurized will depend on the net free vent area. It's conceivable that wall cavities are being pressurized via plumbing and electrical penetrations through wall top plates, and other air connections. To test for this, turn on the fan an remove the cover of a wall switch one of the affected exterior walls. Put your face up close to see if you can feel any air movement.