Where is this method CHECK_MULTI_LEVEL_APPROVER_REQ. Is it in a custom class? Perhaps you can execute it directly from SE24 (if you know the correct parameters - see them from the workflow log) and then debug it there.
If because of some reason you cannot debug the method in SE24 (why?), then go with Rick's suggestions. Here is a nice step by step tutorial how to debug a background step (Of course it needs to be a custom method, so you can add there your own code):
http://scn.sap.com/community/bpm/business-workflow/blog/2015/01/12/workflow-debugging-trick
Kind regards,
Karri