GraphQ

GraphQ[g] yields True if g is a valid Graph object and False otherwise.

Examples

Test for graphs:

GraphQ[Graph[{1 -> 2}]]
(* True *)

GraphQ[{1 -> 2}]
(* False - just a list, not a Graph *)

GraphQ[CompleteGraph[5]]
(* True *)

Conditional handling:

If[GraphQ[g], VertexCount[g], "Not a graph"]

Please visit the official Wolfram Language Reference for more details.