Lots of Rarity

If you have any difficulties with addons, you can use this working macro...

/run for i=1,C_PetBattles.GetNumPets(2) do SELECTED_CHAT_FRAME:AddMessage(C_PetBattles.GetName(2,i).." - ".._G["BATTLE_PET_BREED_QUALITY"..C_PetBattles.GetBreedQuality(2,i)]) end

Just bind it to any key except 1-6 (reserved for battle abilities) and push it during battle

Running the following script ONCE when you login, allows you to see the rarity of the pets you are fighting.

/run hooksecurefunc("PlaySoundKitID", function(Q) if Q==31584 then for i=1,C_PetBattles.GetNumPets(2) do SELECTED_CHAT_FRAME:AddMessage(C_PetBattles.GetName(2,i).." - ".._G["BATTLE_PET_BREED_QUALITY"..C_PetBattles.GetBreedQuality(2,i)]) end end end)

Note if you run this more than once, you'll get multiple messages in your chat frame, when entering a battle.

source: http://www.warcraftpets.com

Even better than a macro is an addon. There's 2 of them I can recommend:

Pet Battle Quality Glow by Emberdawn.
Shows the rarity of all battle pets during a Pet Battle by adding a glow border around the pet icons and coloring the pets name and backup pet health bars. Also works in the tooltip for all battle pets.

rPetBattleAnnouncer (StreetFighter) by zork.
Shows a Street Fighter themed splash screen once a pet battle is loaded. Includes pet rarity display.