The problem is that it isn't that simple. In some cases a bad mod is created by a sloppy author, but in many cases it's a matter of a mod working fine, except in conjunction with certain other mods. Or a mod that works fine on a mid - high end system, but causes problem on budget systems.
Script-heavy mods are fine. But you shouldn't use too many of them at the same time. And the limit depends on your system specs.
Buggy mods and otherwise good mods with "dirty edits" are often flagged in http://www.nexusmods.com/skyrim/mods/6/?with comments, which is part of the reason why people recommend it so often. BOSS also helps to sort your load order so that a group of mods can have maximum compatibility with each other.
Other than that, http://www.nexusmods.com/skyrim/mods/1840/? can help you see issues (with mod dependencies), and http://www.nexusmods.com/skyrim/mods/25859/?can show you most conflicts and records that mods change, so you can get an idea of what a large group of mods are doing, and see which overwrite each other's changes. TES5Edit is very powerful and very useful, but it takes time to learn a little bit about how the game works, and what the records that are changed mean.
The other thing to do is to read through user comments on the nexus before installing (or even downloading) a mod. Often you can get an idea about the stability of a mod by reading through the comments. But you have to be able to exercise some judgement. In some cases, a few comments means that it's just a group of friends giving props to each other. In other cases, a lot of negative comments are generated by sloppy people who don't read instructions and then get angry or frustrated when they break the game due to their own behavior (etc, etc). The comments to watch for are from people who aren't posturing, and who seem to know what they're talking about. If you see a lot of posts like that, you know that something might be up with the mod.
Oh, and with regard to pre-CK software, I always recommend http://www.nexusmods.com/skyrim/mods/26230/?. If you see any mod from Feb. 2012 or earlier, check to see if he's posted a patch for it. There were a few pre-CK mods still getting released after that, but not many.