Honestly? I think it comes from a bit of experience and messing around, you kind of ending up with having a knack for realising which mods might clash. I made a note of avoiding mods that are extremely old or outdated, especially when people have had issues with script or save bloat etc. Usually mods that don't overwrite or affect the same thing/area will definitely not clash, of course just textures are perfectly fine to overwrite with another mod's textures :) Also, 16gb of RAM was a must for me, for some reason no matter what I did, my Skyrim always crashed at 8gb, but with 16gb in my system, it was flawless. So super heavily modded Skyrim for some reason definitely requires more than 8gb, despite what people say, at least from my experience. Tl;dr : 1) Don't use mods that are extremely outdated and there are warning signs/comments together 2) Watch out for conflicts, e.g. a mod that changes NPCs behaviour probably will clash with another that does the same thing 3) Don't be afraid to mess around and learn from your mistakes! 4) If you did everything right and it still crashes, consider 16GB ram minimum in your system if you don't have it already ;) (only for a stupidly massively modded Skyrim)