I find the approach of guessing the assumption or coming up with assumptions upfront like a pretty fun game.
This can get a little tricky with harder GMAT questions but for medium and easy assumption questions it’s a fun logic game.
Another approach that I found helpful was identifying assumptions as I was reading the passage. I would just do it almost automatically. I would read a sentence and I could immediately spot in my mind and assumption that would be required for that sentence to work. Most questions can have multiple assumptions and many of them do but there are certain patterns to assumptions that GMAT follows.
If you run into trouble with a particular question, it’s always helpful to have an example