A lot of smart people trick themselves into thinking they’ve found product market fit. It’s intellectually convenient to convince yourself what you’ve built is working.
It’s good to remember Marc Andreesen’s definition to sanity check.
The customers are buying the product just as fast as you can make it – or usage is growing just as fast as you can add more servers.Marc Andreessen
If you only have a few minutes, Michael Seibel wrote a short analysis of Andreesen’s, along with YC’s take, that’s worth reading.