As someone who sells proxies and maintains a ton of servers let me be brutally honest: you can't be sure.
Unless you have root access to the server, you just can't be sure as an end-user. That being said, you can recognize the symptoms of not-so-private-proxies.
#1 You get banned / blocked a lot while maintaining proper delays. This is a dead give-away.
#2 Load-times are inconsistent. Variance is normal but getting pageloads of 2 seconds one hour and 5 seconds the other is highly suspicious.
#3 You get grey-listed within minutes after using the proxies for the first time. This obviously means the IPs are on some sort of internal list and monitored intensively for any suspicious behaviour.
#4 The silver lining: if providers do that and they get caught it ruins their reputation. That's just not worth it! So in 99% of the cases, if they say they're private, they're most likely private.
I was actually thinking of implementing a live bandwidth monitor for our service once we get enough subscriptions to warrant a full-blown back-end. You solidified that idea ;-)
Hope these symptoms help! I'd be happy to answer any more questions.
