Ask yourself if you comprehend the subject. Do you really understand the topic at hand? There's a difference between knowing answers and being able to comprehend their meaning. For example, critically thinking about a historical topic may not necessarily involve reciting dates and events. It does, however, mean that you need to understand the root causes and effects of a particular historical event.
Try to apply your comprehension to a different topic. Critical thinking is about understanding the links between things. If you understand that two sets of two apples make four apples, can you apply this to oranges, pears or battleships? If you can, then you are thinking critically -- you are divorcing the process from its application.
Look for questioning behavior. The critical thinker takes nothing for granted. Instead, he clarifies a topic, question or issue by asking questions.
Assess whether you are being open-minded. Are you aware of your own limitations, biases and other things that make you more subjective than objective? Critically thinking means being aware of and compensating for the fact that you are not objective.
Look for synthesis. This is the process of assembling and ranking information in order of its importance. If someone is just coming up with a pile of information, then they are not synthesizing. If, however, they can find information and then find the most relevant pieces of that information, then they are synthesizing and therefore critically thinking.