In the early days of a pre-product startup there is little to measure. There’s no sales, there’s no product KPIs, no press releases, and so on.
A good indicator at this stage is product and hiring velocity. How quickly is the product being made into something valuable for customers and how fast is the team growing. It’s a simple heuristic, but potentially useful—without one or the other, there’s a problem (e.g. hiring without product could mean papering over a bad product, slow hiring indicates poor scalability and attracting talent).