He's... not doing a whole lot that would give him away, as he is still Childermass and, like Childermass, can summon up his unreadable poker face at will. He sets the glass of water down and moves on the test the other glasses similarly, one by one. In the end, after he's set all of them down.

"I believe I know which it is," Childermass says once he's done, looking at Strange expectedly to see what he'll do next.