This question applies to any discipline, not just psychology. There are several strategies that can help you determine the discipline of a journal.
If you look up the journal title in Ulrich's, the "subject classification" information will show the journal's primary discipline(s).
Look at the journal's homepage.
Most journals have an official website that explains the scope and discipline of the journal. To find it, Google the journal's full title, it should be one of the first few hits. Ulrich's also lists the official journal homepage.
Find it in LibrarySearch
When you find your journal in LibrarySearch, click on the title to view all of the journal's details. Scroll down to see the subjects assigned to the journal.