There are PLENTY of Intellectual Property guides that can be had for free via the Web. Many lawyer sites cover the various IP law topics (copyright, trademark and patent) along with current court rulings. Or you can purchase a inexpensive Nolo Press published guide on those subjects which are available in both paper and digital. Quite good for the price.
IP law isn't cut and dried. That's why IP lawyers are pretty much the second-highest paid right after criminal defense lawyers.
Companies like Disney, Jack Daniels, etc. not only go after the infringer but the whole chain as well. So, they sue the pirate selling non-authorized goods PLUS the sign/shirt/print shop that made them up.
Everyone should have a hold-harmless form that their customers have to sign if the customer supplies artwork. They attest that they are either the owners of the art and, if not, that they are licensed to use it. And that they will pay any and all legal expenses you incur.
Now that will not stop you, as the print provider, from being sued but it will show the court that you are merely an unwilling dupe and you may get dismissed from the suit after submitting that form to the court. It shows that you do not wish to infringe nor endanger your livelihood by doing so.
You don't have infringe on the big guys to get whacked. Let's say 10 years ago Sign Shop A developed a logo for a client and tossed it in for free to get some good sign work and he allowed the client to use it on his letterheads and business cards only. Today those signs are worn and faded and he gets a quote from you and you are the low bidder. Smart business guy that he is, he has a vector of the logo already to go (sign shop should have NEVER supplied that). Next week Sign Shop A's owner is traveling down the road and sees the brand new signs. Times are hard and there is money to be made in lawsuits. He hires a lawyer, they work out a contingency-fee arraignment and the business gets slapped with a law suit. During discovery they name you as the actual producer. You are added to the suit. You are both guilty and beside any award the court will order for the plaintiff (Sign Shop A) the court will probably award reasonable lawyer fees to the plaintiff as well. So even if you both only get slapped with a $500 payment each (min amt.) to the plaintiff, you are looking at $10k (on the low end) for the plaintiff 's lawyer plus court costs...all this transpires in Federal Court.
There are ways to make yourself judgement-proof but that is a topic for another day.