It’s a survey designed to determine how well your product is resonating with early users, and assess product/market fit qualitatively. Find it here: https://survey.io/
A sample of 40-50 current users should be enough for conclusive results, but the more users you can get to participate the better.

The core product/market fit question is:

How would you feel if you could no longer use [product name]?

And the possible answers are:

1. Very disappointed
2. Somewhat disappointed
3. Not disappointed (it isn’t really that useful)
4. N/A – I no longer use [product]
Plus and open-ended response that would help you understand why.

If over 40% of the users have answered that they would be “very disappointed”, your product could be a “must have” product and that you have found product/market fit.

You want to move the people answering “somewhat disappointed” to the “very disappointed without” group, because these users are already telling you that your product is a “nice to have” product.

Other questions in the survey should get segmented by the choice people made in this core questions. Some other questions that can be very useful:

The question What would you likely use as an alternative if X were no longer available? , can help you define your market and opportunity and learn what people consider as alternatives to your product.

Ask about the primary benefit that the user have received from your product. The answers help inform your value proposition for positioning and discover the core use case for your product.

Ask if they have recommended your product to anyone. You can learn words that people use to describe your product, it helps assess word of mouth for your product, and identify invite and sharing opportunities.

The question what type of person do you think would benefit most from X? indicates the type of people that will value your product and helps figure out market segmentation.

How could X be improved to better meet your needs? With this question you can find out what things people want improved and you can discover things you were not thinking about improving.