I worry that my future spouse is involved in a deviant group or teaching. How can I make sure?

Check carefully before the contract, because a teaching that deviates in belief can harm the household and, in serious cases, affect the validity of the marriage itself.
Allah tells us to verify news before acting (al-Hujurat 49:6), and the Prophet ﷺ told us to choose a spouse for their faith (Sahih al-Bukhari 5090). Signs to watch for: a teacher or group demanding absolute obedience and money; practices without sources and kept secret; claims that only that group is right; or views that make clearly forbidden things lawful. The religious authorities in Malaysia publish lists of teachings declared deviant by fatwa; check with the state religious office. Ask your future spouse directly about their teacher and practices, and involve the family. Do not marry in the hope of "pulling someone out" of the group. See the question on differing religious views.