How to measure heat loss in Tardigrades
1 and 2) You need to build/use a calorimeter : http://en.wikipedia.org/wiki/Calorimeter
Different types exist, depending on your constraints.

3 and 4) It will depend on the size of your system. 

If you decide to use an adiabatic calorimeter: First have an estimate of their metabolism (see e.g. [1]), then divide the energy spent over the duration you think you'll use by the heat capacity of the water volume you have, you get the temperature elevation.

If you use a flux calorimeter, then you don't multiply by the duration but check that the calorimeter will be sensitive enough.