This sounds like an opportunity for the use of this week's Proficiency of the week: Netherworld Knowledge.
Netherworld Knowledge is a Mage or Priest proficiency, that is to say it costs one slot for clerics and wizards, and those with access to such things, such as bards and paladins. For everyone else, it costs two slots. The check is WIS based, and has a -3, so it might not be a good pick for someone scraping the bottom of the wisdom barrel.
Here's your Core Rules definition of the proficiency:
Netherworld Knowledge: With this proficiency, a character learns about the cosmology and organization of the AD&D® game multiverse, focusing primarily on the ultimate destination of spirits after death: the Outer Planes. In addition, the character learns about behavior of the dangerous creatures that inhabit the nether regions, including such fiends as the tanar'ri and the baatezu. As with necrology (which applies exclusively to undead), netherworld knowledge can reveal the specific weaknesses and natural immunities of beings from the Outer Planes. The proficiency can also be used to classify the exact type of extraplanar creature encountered. Both of these abilities require an ability check, however.