There's a note that tells you what the identify text should read, on the "Cure for Lempack" wiki page:
NOTE: Sometimes he goes crazy and attacks you at this point. This is most likely related to the potion you give him. The potion that identifies as "Potion of Disease Warding" is the one you want. If it identifies as "Ochre Potion Disease Ward" or anything else, you'll get attacked.
https://wiki.project1999.com/Cure_for_Lempeck_Hargrin
-Mcoy