Axillary View

The axillary view anatomy: This view provides excellent visualization of the humeral head and neck, though it is typically painful for the patient since it requires shoulder abduction.

Think of it like the humeral head is a golf ball sitting on the tee, which is the glenoid.  If it is sitting on the tee there is no dislocation.  If it is off the tee and towards the acromium, it is posteriorly dislocated and if it is off the tee and towards the coracoid process, it is anteriorly dislocated.