How can we know the binary tree interface? Or what methods does TreeNode have?

For instance, if t is an object/instance of TreeNode, and we want to check if t is root, then which version below is correct?

t.isRoot()

t.is_root()

t.root()

...

and how to access value of t?

t.get_value()

t.value()

t.val

...

which one is correct?