When you build a landing page, do you have a number in mind? Unless X people sign up for newsletter, you will move on to the next idea?
It's far too vague of a question to really respond with a proper answer.
A newsletter subscriber gives an indication you are on the right track, but it doesn't validate your idea. The only real validation are paying customers, doing customer interviews, getting email subscribers, traffic, initial beta users are all positive signs, but really you don't know if you are on the right track until people start paying.
I hope that helps.
That's a good point. Thanks rosiesherry!