Is there another constant or something I should be looking for? I understand how to use say Theta JA or Theta JC and power dissipation to get to the temperature, but what if I don't have the power dissipation is there another method?
No. You have to measure power dissipation in some way.
You can use some hacks: one method is to install temperature sensor (small NTC resistor) to an opposite (to heat sink) side of semiconductor device. Then you have to use some thermal simulation SW (Ansis or similar) to get a temperature difference between heat sink side and the point where your sensor is placed.
For example, Ansis tells that 100 W dissipation causes 20 C temperature difference between the sensor and heat sink. You measure 10 C temperature difference. Then your power dissipation is 50 W.
The junction temperature is then a sum of case temperature and a product of Junction-to-case thermal resistance by 50 W.
This method requires to have an adequate model for Ansis, to install the sensor with good thermal contact, to know case temperature with reasonable precision etc.