Leaking head gasket

Can anyone tell me what the symptoms are for a leaking head gasket? After telling my dealer that my SL320's engine was misfiring occasionally, suffering from terrible gas mileage (less than 10 mpg), and sputtering every now and then (remedy is to shut down engine and restart == problem usually disappears), my service advisor informed me that my head gasket was leaking and that the engine harness had to be replaced -- both of which he said were very large jobs. Given the extensive threads on the M104 engine harnesses, I wasn't too surprised that my harness needed to be replaced (1995 engine with about 55,000 miles). However, I was sort of shocked that my head gasket had failed -- my SL hasn't leaked a single drop of oil into the engine bay or my garage floor since I bought it. Are there any other telltale signs of a head gasket failure (other than dripping oil)?