|Plato and Aristotle, probably|
|discussing the folly of Appeal to Motive|
When you are on the receiving end of the most blatant Appeal to Motive, you can easily counter it by saying something like, "How do you know what is going on inside my head?", or, "Your guess about my motive has no bearing on the validity of what I said". (That is, if you think the attack is worth giving a response in the first place.) Many times, I have seen this fallacy used simply to attack a person instead of engaging in honest discussion. Being aware of its existence can help you keep a cool head and not get wrapped up in emotional distractions and you can get back to business.
On the other hand, when you are seeing or hearing a remark from someone who is questioning the motives of someone else, exercise restraint. It may not be a fallacy if the questioner has some kind of insight about the motives. Also, you may be right about someone's motives based on your own experiences, interactions and evidence. But it may not be a good idea to speak up too quickly, or even to speak up on that at all.
So, the Appeal to Motive fallacy is a frequent kind of ad hominem attack, and you can parry the thrusts of your opponent. But be careful, sometimes it is valid to question someone's motives if you have actual knowledge and want to examine their statement or proposition further.