Legal Question in Business Law in California

Ask for a copy of the promissory note.

If your partner has committed forgery, he or she is less than likely to answer a request for a copy of the note. Maybe, maybe not. If not, go to the lender and ask. If you can't locate the lender, search for the records of the County Recorder under the partner's name, which may turn up a deed of trust that will identify the lender. Although the guaranty agreement itself is not likely to be in the records, perhaps a deed of trust will, and that will show the lender's identity.