Dr. Grimesby Roylott bends the poker through a full 180 degrees, so that it forms a U shape with the two ends parallel to each other. When Holmes later picks up the poker, it is not bent nearly as much, the ends being splayed out at a 45-degree angle.
Snakes are deaf and sense through eyes, smell (tongue) and in particular in feeling vibration on the ground. Therefore a snake can not be dressed to obey a whistle.
After Holmes examines the saucer of milk in Roylott's room, he does not replace the paper that had been covering it. Roylott most likely would have noticed such a change, and his suspicions been aroused.