Measuring the difficulty of vocabulary in a text
The Oxford 3000 Text Checker will tell you which words in a text are not part of the "Oxford 3000", the 3000 most frequently used words. You can type or paste your text and you will get a percentage as a result. The only minor inconvenience is that you have to manually add proper names that you want to ignore. Of course there are other factors that influence the difficulty of a text, but this tool should help you assess the vocabulary level, which is a main factor.