Stephen Pinker, in 'The Language Instinct', provides a rough method.

Use a standard dictionary a pick out a set of pages at random. List the head words from the definitions from those pages in two columns: one for words that you use confidently; the second for those who meaning you can recognise in context. Multiply up the numbers by the ratio of pages chosen to pages in the dictionary, and you have a rough and ready guide to the words you use, and the words you 'know' (even though you may never use them yourself).

I may have misstated the method slightly so I highly recommend reading the book (which is great in many other ways too).

cheer

the sunshine warrior