Actually, you can try seeking out your own brutally honest opinion about your own idea. Act like a stingy investor or a totally down-to-earth common man. You should quickly write down the terse half-line questions that your real childish inner self spits out, before they are drowned by the pitch from your refined outer-self.
At this point, why not simply order an LLM to roast the idea, acting as "the arch-enemy of the pitcher" or something akin to that?