First, you have to know that OpenERP can be very comprehensive when it comes to access rights; you can define access rights at all levels: application, menu entry, view, record, field, workflow action.
Here is an easy way to start with granular access rights (not just the general ones in the access rights tab of the user forms):

• Activate the developer mode
• Every time you want to identify a field, hover it with your mouse and you will see the object name and field name.
• In order to find the access rights related to this field, go to Settings > Technical > Security > Access Controls List and search this object and field. You can also create a new ACL and specify what you want.
• Settings > Technical > Security > Record rules will show you access rights at the records level.
• Settings > Users > Groups will show you the list of user groups and their access rights (trhough the different tabs in form views).