How do I prove that my boss gave me permission to sign a business check

2 years ago my boss was going on a trip to for part business, part pleasure. She invited me along on the trip to see some clients while I was there. In preparing for the trip, she was very busy and I didn't have the money for the plane ticket, so she called me and told me where her check book was located and told me to just sign her name, which I did.The check was over one thousand dollars. Now, her and her husband have fallen on hard times and are saying that I owe them all kinds of money, but most importantly, are saying that I forged the check. This is despite the fact that I flew with them, stayed with them and worked for them while in Hawaii. I was under the assumption that because I was never compensated for the work that I did for them over there, that the trip was my compensation and they never mentioned it again until last month. Is there any way to prove that I was given permission to sign her check?