Yesterday, I stumbled upon LINQ Quiz, try to answer as much as you can from the 17 questions posted there, this will help you evaluate and test your LINQ knowledge.

Oh, and before you start writing LINQ queries against your SharePoint lists, you should take a look at this tool (LINQPad), I found it very useful to refresh my LINQ knowledge before diving deep into LINQ to SharePoint. Let me know what you think about LINQPad, I really love it!