This website offers many tools related to vocabulary frequency analysis, some of which are also useful for teachers and students. This particular tool, Chinese Vocabulary Profiler, helps you analyse the characters and words used in a user-uploaded text. Apart from offering analysis of frequency and showing you which words are in what other lists (such as HSK), it can also show you what words you don't know by way of a user-provided vocabulary list.

As a teacher, I use this tool by uploading a list of the characters and words the students already know, then analyse which words in a new text I plan to use are actually new to them. Please note that word parsing is notoriously tricky in Chinese and that this tool is therefore not 100% reliable. Make sure you save your custom word list in the right format before uploading (GB18030 works well, UTF does not).