Is there a difference in muscle tone between the dominant forearm and the other forearm?
This would depend on how much each arm is used or exercised. If the person greatly favors the either arm, that arm would have more "muscle tone." In exercising, many people exercise both arms equally. This source might help a little.
http://www.members.cox.net/dagershaw/lol/Handedness.html
I hope this helps. Thanks for asking.