Specific vocabulary question: quotidian and tenebrous

Solution 1:

Dictionary.com provides a useful, although rough guide to the difficulty of words in English. Aside from the particular words you're asking about now, the general reference may prove useful to you for a quick assessment of the "difficulty index" of other words.

Solution 2:

If the words' appearances in books are any indication of their residence in people's vocabularies, which they very well may not be, quotidian is significantly more well-known than tenebrous.

Google Books Ngram Viewer:

Considering its proximity to the 0.0% ground level, I highly doubt "most native English speakers" know the word tenebrous.

Their familiarity with quotidian is more up in the air.