Here is some tool which you could check future PageRank, I like IWebTool as they evaluate base on backlinks. While I believe others is checking with 72 other Google Server to get an average PageRank.
This is a predicting tool and may not be accurate. But it is helpful to get a goal in future PageRank.
Note that they are actually facing error currently if you see the picture below. I think Google is changing their algorithm and this predicting tool is having problem connecting to Google Server.
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