Disclaimer: I've never used this tool but it looks interesting enough.
Frequency Level Checker
It assesses the frequency of English words in sentences: level one contains the 1,000 most common terms and their derivatives, level two the next 1,000 words and level three the 800 words most commonly used in higher education which are not found in levels one and ...
Google NGrams could give you a hand. It gives you instances in written texts and that is good for an essay. It is not 100% reliable, but it can provide precious help.
Here you can find information on how to use the site.