General synonyms:
* Perceive
* Recognize
* Detect
* Identify
* See
* Understand
* Appreciate
* Comprehend
* Distinguish
* Differentiate
More specific synonyms depending on context:
* For subtle differences: Discriminate, differentiate, perceive, distinguish
* For understanding meaning: Comprehend, grasp, make out, interpret, decipher
* For recognizing a pattern: Notice, observe, spot, pick up on, discern
* For judging something: Assess, evaluate, judge, analyze
The best synonym will depend on the specific context of how "discern" is being used.