Where do I locate the preprint policies for a journal I would like to submit to?

Policies on preprints vary by journal and/or publisher, but each publication should have information on preprinting in either their 'Editorial or Publication Policies' or 'Author Guidelines' section of the journal website. You might also find these policies referred to as ‘Self-archiving policies’. Some journals may not have this information available, but an email to the editor with an inquiry would give you a definitive answer.