Depend on how much you willing to invest to do the validation.

The methods vary due to many factors such as the nature of business, the ease of collecting data, quality of the information produced, set of assumptions made, etc.

From a simple survey to a structured survey ...
From an empirical approach to a scientific approach ...
From a DIY manner to engage professional ...
From concept testing to sample testing ...
From evidence-based to belief-based...

Should you select one or have a mix of these?

Hope the above help.

If you need more clarification, please contact me without obligation