Yep look for mayonnaise like substance on the top of the oil filler cap or dip stick and/or layer of oil in the water, IE. blown head gasket. But at the rate you are losing water its more likely that you have leak somewhere. Especially because anti-freeze in the water causes huge white bellows of smoke when combusted (like those cars you see that have smoke clouds following them so you can hardly see the road, yeah they have blown head gaskets with water leaking into the combustion chamber).
If you do have a blown head gasket, your engine isn't going to last very long, the anti-freeze when mixed with oil becomes an acid that corrodes engine internals and bearing races. The car will hastily develop a connecting rod knock followed swiftly by a snapped connecting rod, when the connecting rod snaps the piston shoots through the cylinder block and you can pretty much go looking for a new engine (since your crank will probably be messed up, your rods have snapped, pistons will be properly messed up having just shot through the block and probably fallen onto the road, engine block has a hole in it, almost completely new bottom end needed). Whereas catching it early means you only need some engineering work and replace bearings at worst.
