By terrain type and dungeon level, utilizing all of and only the monsters contained in the book, and not those 2-20 tables - which are great for specialized tables, but the failure of the AD&D Monster Manual II is that it doesn't have the comprehensive d% encounter tables in the back like the Fiend Folio.
With the caveat that monsters books containing only a few monsters would be exempt.
With such a protocol in place a DM could easily make up encounter tables like:
d% --- Monster Book Encounter Table
01-24 Human - use Swords & Wizardry City Encounters or other resource
25-49 AD&D Fiend Folio
50 Random Esoteric Creature Generator
51-60 Arduin Trilogy
61-70 OSRIC Monsters of Myth
71-80 OSRIC Malevolent and Benign
81-90 Swords & Wizardry Monster Book
91-00 Tome of Horrors Complete
With the number of monsters listed in each book, an obsessive DM could easily assign d% probabilities according to monster book content quantities.
I think it's great that so many OSR monster books contain indexes with monster listings by dungeon level, terrain type, and so forth, I just wish that they'd take that extra step and tack some sort of arbitrary random table function onto these listings, just for "instant functionality."