Effective way to find and trigger missing/stuck monsters in Doom?

Sometimes in a Doom map, I can't get 100% kills because some monsters just don't show up. I can use the cheat codes to find them in the map, but sometimes they aren't actually on the map they're in a box outside the map and haven't teleported in.

I'm looking for a way to figure out how to get those monsters.

Yes, I could just noclip to them and kill them there, but don't want to do that. I'm okay with using cheats or level-editors to find out how to trigger them, but then load a save-game and do it properly. Even re-playing the level might not help if it's not a bug; some maps are just not designed too well and it's hard to figure out what to do next or it's easy to miss stuff or not trigger monsters, and watching a YouTube video won't suffice because not all WADs have videos of them, and even if they do, they likely won't show the monsters in question being triggered.

I've tried using DoomBuilder to check the LINEDEFs around those monsters, then cross-reference the tags with the sectors and teleport-targets that they're supposed to use to see if maybe there's a specific place I missed (e.g., a sector I didn't enter), but that doesn't always work. I haven't done any Doom level editing since the DOS days, so I'm not familiar with the current state of source-ports and don't know about any new scripting they may have that could be causing the monsters to teleport (or fail to teleport).

What's an effective way of finding them and make trigger them to teleport in?