What do you do if you need to test a system with little to no documentation?

You need to identify what sources of information exist. The common term is a 'Test Oracle'. The Test Oracles will tell you how the system is supposed to work, but unfortunately, they are known to conflict with each other.

This is an old article, but still very relevant on what forms Test Oracles take and what their strengths and weaknesses are.

http://www.softwarequalitymethods.com/Papers/OracleTax.pdf

 

(thanks Peter for remining me that this site has been too silent for too long)