It takes a long time to get really good at jiu-jitsu. Save time by choosing the best instructor you can find.
You’re a beginner, so how can you judge whether you’re getting high quality instruction? An impressive competition record or pictures on the wall don’t make a good instructor.
Talk to the more experienced members in the gym. Have they trained at other gyms? If they’ve trained at a bunch of different gyms, ask them how they chose this gym. Experienced members know what quality training is. Take advantage of their experience. It’s a warning sign if none of the experienced members have ever trained elsewhere.
When you attend your first class, attend the class that you plan to be training in regularly. That awesome one-on-one session may be completely different to the group classes you join up for.
The ultimate way to know if an instructor is good or not, is whether you’re able to perform what you learnt in class when it comes time to roll (live sparring). It doesn’t matter if the instructor can do the move, or if your training partner can do the move. What matters is if you can do the move.
Be pragmatic. If you are having success, you’ve got a good instructor. If you’re feeling frustrated or that the problem is with you, find a better instructor.