If no symptoms, how do I know if I really need new head gasket?

Yes, when possible never go to a dealer for repairs. Shops and local mechanics will always quote a much more reasonable price. Example: I called Nissan for a vehicle I was repairing that needed a front half-shaft they quoted $579 dollars for the part. Autozone had it for $90 dollars. Shop around.

A block test( Hydrocarbon presence) should be able to determine head gasket compression leaks.