My questions is how do I know I'm not getting over charged?
How do I know they're running my ad? I can't check the website all day and they revolve many ads on the website.
1) Get them to point the image URL to your server instead of them hosting it so that you can log each access.
2) Don't worry about it, just measure actual results like conversions and revenue.
If you spend $100 and earn $50, it doesn't matter so much if it's because they underserved the ad or that their audience isn't a good match for your product -- either way, it wasn't a successful campaign so you don't renew.
This is really all that matters. The only thing I'd add is that for you to be able to accurately gauge the success of a given campaign requires that you understand what the lifetime value (LTV) of a customer is. If, for example, you don't know your LTV, then you can make critical errors. Let's say that you spend $100 on a campaign and get 12 signups and $50 immediate money. On the surface, it appears this is a bad campaign - after all, you've lost $50. But if the LTV of each member is $12.50 and you've added 12 new members, you're LTV amount is actually $150 thus making this a $50 profit. Sure, maybe it takes six weeks or five years to get that money but, if your LTV number is close to accurate, then you can rest easy knowing that you should renew the ad campaign.
You could also use google analytics. https://support.google.com/analytics/answer/1247839?hl=en
You could also create a specific landing page that the ad directs to.