How do i know if the hentai game is translated to English?

If it's a Visual Novel then a very handy way of tracking it is via www.vndb.org. Almost 90%+ VN that ever received a full/partial English patch are tagged with a British flag in the release section. On the other hand, if the game is made with RPGMaker software then it's a bit harder to track a localization. You can head over to Dlsite English and track the ones with a translation tag. Finally, if it's a major game that saw release on PS4,X-Box or PC then Wikipedia is your friend. Just google games that have been released in US/EU and you will most likely find a good long list.

If you've never heard of it, it's probably not. The amount translated is few and far between. You can try vndb.org, going to the game and seeing if there's a English release in the tags and language section.