Stanley specifically says that Ansom wasn't attuned to the Arkenpliers. If Stanley didn't know whether he was attuned or not, how would he be able to tell if Ansom was? For that matter, how would he even know it was possible to attune to an Arkentool?

Additionally, in the prose updates on the erfworld site, he instantly recognizes that Wanda is attuned to the Pliers, even saying the she carried them like he did the Hammer.