The best examples we have of a general intelligence are humans.
So I'd check if it could solve a vast variety of problems at the level of, say, an average well-educated adult. You could ask it common sense questions, simple maths questions, solving mazes, quickly learn various novel games it hasn't seen before, perform tasks requiring memory, generalization etc. etc.
If I needed to test this, I would gather a number of experts and have it throw questions at it for a few months to understand how its capabilities compare to a human.
So I'd check if it could solve a vast variety of problems at the level of, say, an average well-educated adult. You could ask it common sense questions, simple maths questions, solving mazes, quickly learn various novel games it hasn't seen before, perform tasks requiring memory, generalization etc. etc.
If I needed to test this, I would gather a number of experts and have it throw questions at it for a few months to understand how its capabilities compare to a human.