There are a couple of ways to determine the menu items and web menu items associated with the workflow.
The first way is to go to the AOT and look under the Workflow > Approvals node
The required display menu and web display menu items are highlighted below
The required action and web action menus are highlighted below
Check that the user has permission to access these display and action menu and webmenu items.
You can also use the Visual Studio 2010 debugger on the SysWorkflowDocument.assertAsUser() class method to locate the menu item(s) that the user doesn't have access to.