The “books” which should consist of all checks written/ deposits etc. would show if you are paying someone “under the table.” They usually go through all of the checks written to be sure you didn’t pay a subcontractor, or in your case your GF. You should really keep some kind of records- how do you do your taxes with no records?!?!? You sound worse than my clients at CRC!!!
To be honest if they come in and you have no kind of records you are opening a can of worms… IF you pay rent you are required to send your landlord a 1099 at the end of the year (assuming they aren’t incorporated).