1) Apprehensively 2) Askance 3) Charily 4) Disbelievingly 5) Distrustfully 6) Dubiously 7) Guardedly 8) Hesitantly 9) In a doubtful manner 10) Questioningly 11) Sceptically 12) Sideways 13) Suspiciously 14) Uncertainly 15) Unsurely 16) Warily 17) Without conviction 1) Disbelievingly 2) Dubiously 3) Hesitantly 4) Implausibly 
• (adv.) In a doubtful manner.
Doubt'ful·ly adverb In a doubtful manner. « Nor did the goddess doubtfully declare.» Dryden. 
[adv] - in a doubtful manner
dubiously adverb in a doubtful manner; `Gerald shook his head doubtfully`