Have someone lay on the bed and watch it lose its air while going around it like you would with a fine tooth comb? Then find it and patch it with a quick repair kit or use some good ole fashion duct tape?
I wish I could remember the trick. Someone shown me how to do it by putting about a cup of water, and I think mixed with soap, inside the airbed. Then they found the leak that way. It was pretty neat how they did it.