Coolant Vani****ng: '95 Civic EX 1.6 - Attn Tegger and Techs

When I had the oil changed in November, I was told they had to add 
"quite a bit" of coolant to the reservoir. Since the car came from NM, 
with a spotty service record, I had hoped that it was simply not topped 
off ever. Now, three months later, with only moderate driving, I see the 
reservoir is virtually empty again. I don't see excessive white smoke in 
the exhaust, or smell antifreeze in it - although I haven't sniffed it 
specifically for that, yet. The oil looks fine. I don't smell coolant 
inside the car, either. It would be hard to spot a slow drip in this wet 
Winter weather. How common are internal head gasket leaks in this 
engine, as opposed to slow leaks in the reservoirs?