Here is a pretty extensive interview featuring Sean Ellis on getting Product/Market fit. If you have the time go and listen or read it. If you don't here you can find the main takeaways:
1. There is little point in using the full force of marketing to scale your startup unless you achieved the product/market fit. In other words you first build something that people actually want. Then you market it. In that strict order.
2. The recommended way to determine if you are there is to survey your early users to determine how disappointed the would be if your product/startup dissapearred today. You are looking for very disappointed people. Somewhat disappointed doesn't make the cut.
3. The magic number is around 40% of very disappointed people. If you get to this point, you got your product/market fit.
To me, seems like sensible advice. Although a bit simplistic (I can imagine many ways to get it wrong such as surveying the wrong set of customers or just not being able of crossing the chasm between early adopters and the early majority) , it's a good set of heuristics to keep in mind.
Update: It's worth to mention that, in MBA-speak, what you are indirectly measuring in point 2 is your added value. The sole mention of "Added value" has always turned on my fluffy buzzword alarm, and it probably does the same for you. But in the end is quite simple: the added value of X is the difference between the world which includes X and the world without X. By seeing it that way you can easily see why Google has so much added value (try to live a week online without using anything from Google and you will get a little sense of how the world would be if no one could use anything from Google). And you can also see why the typical random startup has not much added value.