A check can be used as evidence of a debt. However, keep in mind that a check can represent a gift, a loan, or an investment. As such, don't assume that the check is tantamount to a promissory note or a written contract for the repayment of a loan. Therefore, it is best that you have a promissory note to reflect the terms and conditions of the loan.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.
You should use a promissory note. If the debtor does not pay you as and when agreed, you will need to present proof of his/her agreement to pay a certain amount on a certain date. Another words, the legal obligation is the promise to pay you. The check only establishes that he/she received money from you, not that he/she agreed to pay you back.
Consider going online to a free lawyer document site like docstoc and get a promissory note template off that site to get your friend to sign. Also many states require the promissory notes to have documentary stamps or doc stamps attached to them to be valid. Consult a local CA attorney. Also consider a simple contract which you can also pull off that same site just to memorialize the entire scope of the deal in writing. Good luck.