Pre-seed startup beta testers

I became part of a small startup via engagement on LinkedIn. We have now developed a basic minimal viable product (MVP) in the form of some code that I run on my computer. It is a niche service that compares the environmental toxicity of chemicals. We want feedback to base our second product development iteration on. My question is if any of you have experience with probing early stage services and how you got the feedback you needed and translated that into getting paying customers, or if you were on the other end of the bargain and can share some insights.