Here's an old trick which can tell a lot. Even though it can appear difficult to understand clearly without actually being there. And not to mention my mother tongue is french. But I'll try anyway.
Ask someone like your teacher to flip your hand/wrist/arm away while you're playing. All of a sudden. Without warning (that's the tricky part).
If your hand/wrist/arm fly away from the keyboard, your technique is OK.
If your hand/wrist/arm stay locked on the keyboard, you're too tense and/or not using the right set of muscles. For example, if you play too much from the hand, your hand/wrist will be locked to the keyboard when the flyaway wrist test is applied.
If you feel tension or pain in the top forearm muscle, chances are you're playing too much from the hand and/or fingers alone and/or using too much effort.
For example, in Chopin's Fantaisie Impromptu
, a pianist will feel exhausted playing the score at the written tempo before the finale, if he/she plays too much from the fingers. You need to release the tension in the fingers by using the available weight in your arms. In other words, use your fingers more as pivots/fulcrums (not sure how you say that in english) with your whole forearm and arm weight being the available power source to help you play with minimum effort.
I'm sure you already know that, but this test can remind any pianist if he/she is too tense. Good to use before a concert.