Adrian reviewed our application at a time when we had absolutely zero evidence of traction or growth. Our product was (and still is) unfinished and certainly not at the stage where we could run beta tests.

To combat this, we presented a concept video on our website[1] and accepted emails for signups. By carefully explaining the idea and finding people who might be interested in it, we've managed to collect over 600 emails in the past few weeks. I'd recommend a similar approach, reach out to people and communities who might be interested in your product. If they're unwilling to give you their email, it might indicate they're even less willing to give you their money.

Once we flushed the idea out a little bit more, we updated the video[2] with actual footage.

[1] https://www.youtube.com/watch?v=nbezr3yDJtQ

[2] https://www.youtube.com/watch?v=CuQ8NJOypqs

You can't really have growth without a product, but like PG said in the top comment, that's not a reason not to apply to YC. You can probably still assess demand - hopefully because you yourself want what you are building. That's also helpful for us to say, though again not make or break.

Growth and potential market are things we'll look at, but we're careful about making sure neither unduly influences us. The most important things are the founders and their relationships to one another. We want great founders with great ideas and the tenacity and ability to go after them.