For a single body, it is seldom difficult to see whether it is fully constrained or not, but for a more complex assembly, including several physics interfaces, or advanced couplings and boundary conditions, it may not be trivial. If you suspect that rigid body modes is a problem in your model, you can run an eigenfrequency analysis, and check for modes with zero eigenfrequency as described in Eigenfrequency Analysis.

If there are no constraints which are dictated by the physical boundary conditions of the structure, you can use the Rigid Motion Suppression boundary condition to automatically remove the rigid body motions. When you do this, the assumption is that the external loads are in equilibrium. If not, reaction forces and stress concentrations will appear at seemingly arbitrary points where the automatic constraints were placed.

As an alternative to applying constraints, you can also add elastic supports through a Spring Foundation node to suppress rigid body motion.