It's always difficult to "prove" fraud but the best you can do is show reputable data logs of the traffic. So in your example if you are (hypothetically) being billed for 100 clicks but only see 55 visitors then export or screenshot your analytics and send them to your account rep. Programs like Prosper202, GetClicky, Google Analytics or your server logs are reputable.
You also mention the fraud could be taking place via meta refresh. Meta refreshing does not get rid of referrals 100% of the time. It only works about 90% of the time at erasing the referral. So you should be able to view your referral logs, identify some referring sites and visit them to check for anything suspicious.
You also mention the fraud could be taking place via meta refresh. Meta refreshing does not get rid of referrals 100% of the time. It only works about 90% of the time at erasing the referral. So you should be able to view your referral logs, identify some referring sites and visit them to check for anything suspicious.