I wonder if there is a "general rule", a kind of "algorithm" for finding the components of the Stress-Energy tensor in for particular cases. For the Einstein tensor, just by knowing the metric, one can find the components of it. What about the Stress-Energy tensor? One way I thought of (and that is the way I have been encountering in books on GR) is using what we know about the scenario... to make approximations and then deriving the components. Another way I thought of is by using variational principle. Would this require knowing the field that describes the matter in the situation we are considering, right? Is knowing the field too hard?