A graph is validated whenever you save it (File 
Save or Ctrl+S), request validation (Build Validate), or evaluate it (evaluation requires you to save first, anyway).
A graph can be invalid for a number of reasons:
- You are using inputs and the graph is not a compound.
- You are using parameters and the graph is a tool (not a compound).
- There are no output nodes or multiple output nodes.
- There are unconnected inputs that cannot be interpreted as function arguments.
Unconnected inputs are detected by traversing the graph in reverse direction, starting from the output node.
- There are type mismatch errors.
- A deprecated operator is used.
- An illegal operator or compound is used.