General:
* Perceive
* Detect
* Recognize
* Identify
* See
* Notice
* Appreciate
* Understand
* Comprehend
More specific:
* For subtle things: Discriminate, distinguish, differentiate
* For knowledge and understanding: Grasp, apprehend, comprehend, fathom
* For insight and judgment: Judge, assess, evaluate, analyze
* For seeing through deception: Penetrate, pierce, unravel
The best synonym for you will depend on the specific meaning of "discern" in your context.