General synonyms:
* Curious
* Inquisitive
* Skeptical
* Doubtful
* Puzzled
* Questioning
* Perplexed
* Amusingly puzzled
More specific synonyms:
* Wry (implies a dry humor or amusement)
* Scornful (implies a hint of mockery)
* Sarcastic (implies a mocking or ironic tone)
* Suspicious (implies a lack of trust)
* Intrigued (implies a strong interest or curiosity)
The best synonym for "quizzical" will depend on the context in which you're using it.