How do I critically evaluate a theory or field of psychology? (e.g. biological, behavioural, cognitive etc)

You must first make sure that you understand the main assumptions that are in line with the given theory. Then you should find studies or theorists that support them, and be able to explain how their findings relate to the assumption. You can then begin to critically evaluate these studies in terms of their validity, reliability, generalisability etc, and perhaps use other studies to compare/contrast to support your evaluation. Then you can do the same with the assumptions of the theory as a whole. Do its methodologies provide valid empirical datat? Do they provide effective treatment methods?

After critically evaluating the back and forth with the pros and cons of a study/theory, it is always important to round off with a conclusion or statement about the general view that you hold about it (without explicitly stating that it is your opinion, as it will be informed by the arguement that you have presented).