Check the tailpipe when motor is running for water droplets coming out, a sign of a blown headgasket.
Check the oil for moisture/condensation, light brown foamy oil.
How hot is it running?

There is a 1" concave freeze plug at the top rear of the block that is inaccessible with the engine in the car, as it is by the firewall. These often do spring a leak after corroding. Look for a drip at the rear of the engine when parked with engine running and fully warmed up.

If parking on a cement drive should see a wet spot when loosing a gallon, however, if parking on gravel slide something underneath to help pin point location of a drip. Another possibility if you regularly refill fill to just below cap it may throw out some water to get a lower level in tank for expansion. If suspect leaking into p[an let set overnight then jack (and block up) car enough to slide underneath then use a clear plastic Jar to drain a little oil from pan..([u]Do this over a regular Drain pan[/u]) If water/antifreeze in pan it usually drains out first then re-install the drain plug. If water/antifreeze didnt appear at first let the Jar set overnight to see if water separates from oil.