I figure anytime you see someone use a non-Western-US accent (that isn't from the Western US), they are speaking English; otherwise, UT: folks like Picard, Papa Sisko, the Rohzenkos, etc. were speaking English, as those accents come from regional pronunciation. Possible exception for the founding Fed races (especially Vulcans), and the occasional Klingon, since we see them speaking English before UT was invented. Though maybe there was a primitive one in use by then, which didn't work completely by magic, since the Enterprise crew was often able to communicate in places that would probably never heard a word of any Earth language, let alone English, though not always (ie, it was already programmed with several known languages).
Also, it seems the UT works at least with AoE - in LGM once the Ferengi got their UT's working, everyone was able to understand each other, and it made it look like
they were speaking English to the A51 guys.
Biggest problem with UT as shown on the show was Klingon. Why would they randomly have Klingon words get through the UT? It's not like all of it was untranslatable, if any. And I think at least a few times it happens, the Feddie characters don't know what's going on, like the UT stopped working.
This could also explain some of the lazy cultural stuff naming - stuff like the SPACE fleas, or SPACE pudding, etc.