You want to use a phrase (or combination of phrases) that should only appear on your page, and nowhere else on the web… or very few places at least. Then you do the search - if there's more than one result (your page), then you need to examine the other URLs that are listed. If some of them are delivering an exact copy of the page, you just may be dealing with a proxy that has hijacked your content.
www.example.com/nph-proxy.pl/011110A/http/www.mattcutts.com/blog/
It's easy to see what URL that would fetch, if example.com were a real proxy. Other proxy URLs encode the target URL so it's not always that easy to determine what they're going to fetch just by looking.
The mere presence of proxies in the index doesn't necessarily mean you'll be dropped or penalized. The situation inside Google's systems is no doubt very complex. I have seen sites with multiple proxies indexed, and no ill effects. It's possible that there are certain factors (trust, authority, domain age, etc.) that make one site more susceptible than another. I have no idea how they make the decision on which copy of a page to keep.
If you discover that you have a problem (pages knocked out of the index, -999 "penalty"), and you can identify proxies as duplicate content, a reinclusion request is likely to work in the short term, while you implement countermeasures. If you don't mind sharing your information with me so that I can use it for further research, send an email to proxyreports@gmail.com with the affected URL and the URL of the search result page that shows the proxy duplicates, along with any search terms where your ranking appears to be affected.