How do you measure user/customer experience for an in development product?

This may at first sight seem an overly broad and basic question. But drill down a little.... its really not so simple.

Measuring usability- easy. Lots of different ways to test and measure the usability of a system

Getting qualitative input about user experience- also pretty easy.

Measuring overall user experience however, for something that is not even being actively used yet- is this at all possible?

Certainly for an already in existence product there are methods. Customer satisfaction surveys, interviews where you ask about parts of the process that annoy them, etc....

But for something which has not yet been launched... is there any way to get a solid, scientifically reliable measure of its user experience?

Does it require having a past version to use as a benchmark? Assuming one has this, what method is possible here?