It's not hard to recognize when you have a good product when it's feature complete and selling like hotcakes. Recognizing that you have the core of a good product with potential to be great if you slog it out and stick to your vision is hard. Really hard.
No doubt, but if you have Steve Blank as a mentor telling you that you have a good product it might be a significant clue.
Also, potential customers flat out telling you that they are ready to pay. That kind of helps.