I would put it this way: Demonomicon was pretty much the book which came out right before MM3. I think some of the monsters in Demonomicon could easily be considered on par with MM3 designs, and other ones not so much. Its even less clear with stuff which appeared in Dragon, which may have had a long lead time. Some stuff produced by people who were aware of the general updates to monsters could have appeared earlier than MM3 and embody the same concepts. Just as easily there's stuff which appeared quite a bit later which clearly does not.
Beyond that, a lot of even early monster design is quite solid. MM1 orcs for instance are pretty solid monsters, kobolds too. Probably more than not the lower level monsters are pretty good (hobgoblins, meh, Spectres BLAH!). I don't think every monster is great simply because it is in MV or MM3 either!
Anyway, if you think a monster falls below the damage output guidelines for later monsters, just tweak the damage expressions, that helps a lot with a great number of monsters.
The best giveaway, in theory, is the statblock format. The new style statblocks are pretty much guaranteed to be modernized monsters and usually the quality is better. Definitely the format itself is a bit nicer.