Any time you are receiving art from the client, your contract needs to state and they need to sign off saying that they own or have permission to use the provided art work, there is no way to know or research every piece of art work that comes in. But also like stated above, some art work is a no brainer, mickey, spiderman wonder woman etc. the people who do pay for rights to print these characters will have a disclosure letter saying so, this goes for sport teams as well.
I would turn the work down a law suit is not worth the few bucks you would make.