How do you validate an idea before writing any code?

Building something for days only to realize nobody wants it is the worst feeling.

I used to jump straight into coding every cool idea but now I try to test demand first with a quick landing page or simple mockup.

Talking to users directly helps but people often say they want a feature and then never actually use it.

Do you look for active complaints from current users or do you pre-sell ideas before starting development?