If I am looking at a quest in e.g. tes4edit, how can I tell the difference between a quest designed to appear in your journal and be played vs a quest used internally by a mod or scripting or whatever that never actually appears in your journal or becomes completable?

Well a quest which appears in your journal is really easy to identify, as it has journal entries, which you can see in TES4Edit or in the CS(E).

But there are also non-journal quests, those are harder to tell apart from quest records being used purely for scripting or dialogue reasons. If the quest record is not being referenced by any dialogue TOPIC/INFO records, then it is probably not a playable non-journal quest - but even that isn't a sure sign, as there can be non-journal quests where the player does not need to talk to anybody, just follows a script-triggered clue-based path to completion. And then there's the reverse, a quest record may be used to provide non-quest dialogue, so it will be referenced by TOPIC/INFO records yet is not a non-journal quest. Really, you need to look through the entire quest record, and check every referencing record and figure out what exactly the quest record is for.