When something is not in order for a node, it exhibits an error or a warning underneath its main body:
Errors indicate that the node is unable to calculate its outputs, while warnings simply indicate possible oversights that may lead to surprising behaviour.
While different nodes have different errors, all nodes (except plot) have input errors/warnings. Input errors indicate a required input is missing, while warnings indicate a missing optional input. The names for the inputs shown do not always correspond to the labels, but should be understandable from context. "a", "b", and "c" always refer to scalar inputs, in ascending order, while anything named "$\psi$" refers to a state.
Nodes that allow writing a math-expression have errors looking like "parse error matching struct...". These errors indicate a bad expression. For a reference of allowed Expressions, see the Expressions-page.