If you can get the car up to N.O.T (normal operating temperature), the sure fire way would be to carry out a cylinder leakage test.
This will determine what cylinder is your problem cylinder, wheher it be in the block or cylinder head.
Even if the leakdown test revealed anything or not, then your next step would be to cary out a cooling system pressure test.
If after you have done these tests, you find that you have either air going into your radiator, or coolant into your cylinder.
If there is coolant going into your cylinder it doesn't necessarily mean head gasket, especially if you have just done one.
If you are not confident on carrying out these tests yourself, then i would take it to a reputable mechanic who you know will look after you.
Are you sure that you don't have an airlock within your cooling system? Airlocks can often make a simple problem look more dramatic than what it actually is.