Try this:  For any given user account (or provably grouped user accounts), define
normalised shares = each share divided by the the difficulty at which the share was submitted
X = sum(normalised shares)

Then for one block being solved by this user account , the lower tail CDF is:
CDF =  1 - exp(-X).

Notes:
1. If the user account has never solved a block, then the CDF will be greater than this when or if eventually they do submit a block.
2. If the user account has solved more than one block, you can use Wolfram Alpha to calculate the CDF - there's a howto in point 6 of this post: http://organofcorti.blogspot.com/2015/07/faq-bitcoin-mining-and-luck-statistic.html



They're a proxy, so I think you have to believe their stats. You could use the method I gave above to check the CDF though.