Product vs. Execution

As a new founder, I have read a lot of books and guidance that suggest just getting your product 'out there' to see if it's a good idea or if people are interested in buying it. But my struggle with this is unless you have a strong product / marketing / pr person on your team you could assume people don't like the product, but it's actually a great product and the issue is marketing or messaging. For example, maybe I'm using the wrong channel / medium and it's just not my audience. Maybe the product is great for my target audience, but the messaging doesn't properly explain what we do or why they should use us. Self-funded, so we don't have unlimited funds to run tests, but we've worked with a few consultants that have helped, but most of our learnings have been trial and error. My bigger question is, how have founders - especially technical founders who like me aren't particularly skilled in GTM - evaluated product / market fit? And what are realistic KPI's? Is it free trial signups? Website traffic? Paid customers? Email clicks? All of it. How do people know when they're on the right track and when something needs to change? I want to pivot when it makes sense to pivot, but not jump the gun so I'm flip flopping every week before giving something a chance to succeed.