Is it possible to check a journal entry from a quest which is added by another mod?

I want to check a condition from Great House Dagoth

Create the journal entry with the same name. Then you should be able to use "GetJournalIndex". You have to keep the "Info/Responce" empty, just to be on safer side.

Thank you

Would this mean that my mod will have to load after or before Great House Dagoth?

It shouldn't matter as long as you don't have any actual journal entries. Just having the journal should be fine. You can run into problems if you start populating it though. You can fill in the info/response in dialog, but don't add anything to the journal itself. Otherwise your mod will have to load before so GHD will overwrite your changes.