Hi mate,
I understand you are concerned about the mayo you have discovered on your oil filler cap. However as stated in your other thread I am pretty sure this is probably down to moisture build up due to you doing short journeys. You are not losing coolant (2-3 teaspoons in a year is nothing to be concerned about at all),
The main symptoms of head gasket failure (amongst others) are
Symptom #1: Chronic Engine Overheating
One major blown head gasket symptom is a consistently high temperature of the car engine. This is a symptom that can be easily checked from the temperature gauge of the car dashboard. So, chronic engine overheating is both a cause and symptom of a blown head gasket.
Symptom #2: Significant Drop in Coolant Level
Check the engine coolant level. If it has dropped suddenly, then there is a possibility that the head gasket has blown up.
Symptom #3: Mixing of Coolant and Engine Oil
Check out the engine oil of your car. If you notice a frothy formation on the dipstick, then there is a possibility that coolant and engine oil have gotten mixed due to a blown head gasket.
Symptom #4: White Smoke from Exhaust Pipe
If you notice emission of sweet smelling white smoke along with water droplets from the exhaust pipe, then there are chances that a head gasket has been blown off.
Symptom #5: Popping of Coolant Overflow Tank Cap
Let the engine rest and cool down for about twenty minutes. Then open the car hood and locate the coolant overflow tank. If you remove the tank cap and it makes a popping sound, that means the cooling system is still pressurized even after engine has shutdown. This is a clear blown head gasket symptom.
Is your car suffering from any of these symptoms? If not then it is almost certainly an issue with moisture build up. However if you are still not convinced about this I would pop down to an indie and ask them to do a sniffer test. This will give you a more definitive answer. Somebody may post up a picture of a lovely clean filler cap, but this doesn't mean your headgasket is failing mate!