Application

A good instructor should be capable of demonstrating tai chi self defence without hurting you.
They can demonstrate striking power on a focus mitt.

Do not be afraid to ask questions.

Gauge the effectiveness of what they show you:

1. Did it work?
2. Did they compromise themselves? Were they over-committing?
3. Was there any adverse feedback?
4. Did they allow for multiple attackers?
5. What did it do to the opponent?
6. Were they forcing an outcome? Or did it flow?
7. Was it easy to perform?
8. Smooth or jarring?
9. Was it hurried and quick? Were they calm and composed?
10. Can they evade an armed opponent?