In both 1st and 2nd edition, the DM had to do rather more work to determine an adequate challenge*.
What I always did was read up on various sorts of foes, and then you just.. decide if it's an adequate challenge. Sometimes you were right, sometimes you were wrong. Determining challenges, then, were much more dependant on how much experience the DM had at doing it. Of course, there are lots of great big signs to help you.. for example, hit dice are good, special abilities of the foe in question, etc. It's easy to tell that a beholder requires a higher level party, etc. You could also check out and see how much XP a given foe was worth, too. As an example, checking out my 2nd edition Monstrous Manual I happen to have here... you get 2000 XP for killing a lurker -- so 1 lurker would put just about anyone to level 2 after 1 fight. Without even checking the HD (which happens to be 10), I would guess that most lurkers would kill off most 1st level parties real easy-like, but a good 5th level party wouldn't have too much trouble. On the other hand, a kobold is worth 7 xp. So you can tell just from the xp value that you'd need a good number of 'em to challenge a first level party.
In some ways, I preferred this system -- Life isn't very mathematical or cut and dried, and sometimes bad things do happen to good people.. On the other hand, killing the party because you misjudged how tough was some monster is no fun at times. Depends a lot on the campaign, of course. Mighty save-the-world quest campaigns go nowhere fast if everyone gets killed by a manticore at level 1.
*Sometimes you have to do a good deal of work with the CR system too, but in the main it's easier, since it's a nice formula.