Question

how does the snake appear to be harmless

Answer 1

milk snake or milksnake,  is a species of king snake. There are 24 subspecies of milk snakes. Lampropeltis elapsoides, the scarlet kingsnake, was formerly classified as the subspecies L. t. elapsoides, but is now recognized as a distinct species. The subspecies have strikingly different appearances, and many of them have their own common names. Some authorities suggest that this species may be split into several separate species. They are notvenomous (or otherwise dangerous to humans).

Answer 2

The snake appears to be harmless.This statement is evident from the lines "he lifted his head from drinking,as drinking cattle do".  This line supports the fact that the snake is harmless just like the cattle. Even when the snake became aware of the narrator's presence, it thought for a moment and continued to quench its thirst. If the snake was dangerous, then it could have attacked the narrator but it didn't do so. Hence it is harmless.