You need them to provide you with proof of trademark use, and then have them fill out an Intellectual Property Rights form, otherwise known as an IPR, and have it notarized. If you already have an attorney, they can do a quick back ground check on the individuals, the TM and draw up the proper paperwork for you. We deal with a few Record labels that have rights to produce Merch for their artists, but only certain designs and only in certain regions. So we do this quite often if it's a project, or even a re-run of a project, since sometimes there are time limits on the use of Trademarked property. If someone's trying to pull a fast one, you just won't hear back from them. The should be very forthcoming with such documents if they produce merch already.