Assuming this is a search campaign, you should be able to see if any new competitors are regularly showing for your keywords. I would also investigate if CPCs have changed (or just conversion rate), how your impression share has changed, and how your average position has changed.

If their campaigns are profitable, they will probably continue to run them. Your best bet in the meantime is to optimize your CPC, CTR (helps with CPCs), and conversion rate. You have the history and experience, which should be to your advantage against a new competitor.

There are third party click fraud and forensics companies that can investigate this for you. You can also call Google (866-2-google) and ask them to investigate...
In the mean time you could block specific IP addresses in your AdWords settings to block your competitor from clicking on your ad...

In general, unless you are constantly optimizing your CPAs will naturally go up as more advertisers enter the auction for your keywords. Take a look at your impression share report and you historical ctr's to see if anything changes drastically....

In general, Google is pretty good about detecting click fraud and not charging you for it, but it can't hurt to verify...

If you need deeper proof you can match up your serverlogs to AdWords reporting and show any discrepancies to Google.

Good luck.

I am not an expert on CPA advertising. But, I am surprised that your advertising provider does not have a fraud protection mechanism in place. Have you reported your suspicions to the agency? I'm sure you have. I'd be curious where you are advertising and what they told you. Drop me an inbox message and I'll do a little research for you.

Best regards,

Kevin McCarthy
www.kevinmccarthy.com