this situation came up earlier tonight. I had a goyf on the board and my opponent 'evoked' his shriekmaw. I didnt know if evoke was an activated ability and I could stifle it. or if it was actually being cast, in which case I would force it. I thought that it was being cast and I could force it, though I had stifle in my hand too. I was up at the front of the shop, so we asked one of the owners. he looked it up and said it was a static ability, so I couldnt do either.