There's a big step for startups to find a product market fit, is there something similar for research ideas? I can only think of talking to your PhD advisor, or applying for small grants like https://manifund.org or https://experiment.com/, are there any other options? Is there a place online to discuss research ideas before working on them?
There might be but instead of thinking about it as a binary, think about it as a continuous conviction curve. Ask yourself, if this idea is true, what's the smallest testable prediction you can make? It can mean running a small experiment that doesn't give you statistical significance or would pass scientific scrutiny, but can be a gating function for a slightly more sophisticated experiment. And then you rinse and repeat until you prove/disprove the idea.
One idea maybe to submit to an industry conference and review the feedback. Usually they only require you to submit a short abstract and the turnaround time is a few days.