At this early stage, I like to focus on experimenting with funny head shapes to figure out which direction you want the exaggeration to go. And then whichever thumbnail sketch has the funniest shape while showing the most potential for a good likeness is the one you should take to the next step. Likeness is a part of the decision at this stage, but I don't think it's the most important yet. Not all caricaturists work this way, but I think it's a good way for people new to caricature to break down the process into more manageable steps.