Gravity.

Since you are not going to be able to heat the water to a boil, use a funnel and a stopcock valve on the top.

You probably won’t get a true reading unless you have enough water to completely fill the system, so you’ll also need a stopcock on the bottom.

Measure your change in temperature of the H20 over, say, 5 minutes. (Fill system, wait 5 minutes, drain system and measure temp in outlet flow.)

And, if possible, use copper – better heat absorption.