If your tai chi chuan teacher has groundpath, you should be able to put your hands on their arms at any point during practice and ‘bounce’ their structure.
When they are halfway through form, ask them to freeze. Then test their posture. Is it substantial? Is it stiff? Are they tensing-up against you?
Be vigorous. Feel for gaps and weaknesses.
They should be springy like elastic, yet internally connected. They should feel relaxed but strong, with absolutely no conscious effort required.
If your teacher crumples in a heap or pushes back into you, find another instructor.