Following script and get all the packages related to API in Oracle applications, from which you can select APIs that pertain to AP. You can change the name like to PA or AR and can check for different modules
, substr(u.status,1,10) Stat
, substr(text,1,80) Description
from dba_source a, dba_objects u
and u.object_name = a.name
and a.text like '%Header%'
and a.type = u.object_type
and a.name like 'PA_%API%'
Oracle Database 11g The Complete Reference (Osborne ORACLE Press Series)