2008 Chevy Aveo Verifying Head Gasket Problems on a 2008 Ch

My wife purchased a 2008 Chevy Aveo a little over a year ago. This past winter we noticed that the heater started cooling off when idle. I checked the coolant and the reservoir was completely empty. I filled the reservoir with coolant and began looking for possible causes. Though I didn't notice any problems initially, a few weeks later a small hole developed on the upper radiator hose and blew out completely the next day. After replacing the hose and filling with coolant again, I continued to notice a sweet smell and a continually emptying reservoir.

About the same time I also started noticing the engine knocking or sputtering when it was started in the morning. Though not concerned at first, when it continued every morning and I was surrounded by a cloud of white smoke when I backed out the whole event became suspicious.

I told somebody what was happening: the engine knocking when started, the white smoke, and the continual need for more coolant; and was told it was probably a blown head gasket or similar issue and I should take the car to the dealership since it is under warranty. Now in the present, the car has been sitting at the dealer's repair shop for three days and they can't seem to find any issues. They noticed that the coolant was low, though I had filled the reservoir the night before. They've run a pressure test and started it multiple times and continue to say that they haven't noticed any white smoke, knocking, or cause of antifreeze leaks. They've told me to come get the car and come back if anything else happens. Since the knocking and white smoke have happened every morning and "cold" start for several months, I'm thinking that the issue shouldn't take long to surface again.

So for a few questions:
What is the most likely cause of the problems listed?

What tests can "prove" there is a problem when the only answer I get from the mechanics is that they can't find any problems?