Several ways come to mind.

Formulate your idea into one (or better all) of the following + you'll know by interest level if you have a winner.

1) My favorite. Start a Meetup Group (set the zipcode of group) to match the zipcode of a US city where most people originate travel to your target destination.

For example, If your targeting running tours in Chang Mai Thailand, research what city people in US most travel from to visit Chang Mai.

Then run Meetup events related to Thai Travel topics.

2) Package information about Thai Travel into a PDF + sell it via Facebook Ads.

3) Start a Kickstarter project.

4) Speak at existing Travel Meetups + other groups + related conferences about your idea.

What pain points fixed by your solution have you identified that your target market acknowledges?

http://www.salestactics.org/pain-points-back-basics/

Will anyone pay you money for your solution?

And yes, you can sell it before you've made it. You're customizing it for them, or giving it to them at a discount, or both.

That's validation.

There is success and failure in every industry and every market. No Idea is a failure or success. I would evaluate the person responsible for launching said idea instead of the idea itself.

An Idea is but a seed, you have to first find a farmer that specialize in growing that crop, then the farmer will let you know if the seed is damaged, or if it is the right time for planting.

So the short answer is to bring your idea to a farmer.

One way is to create a landing page with the option for people to include their email address if they want to be updated about the product/ service when it's available. Apps often do this while they're creating the product.

I do idea validation. Feel free to have a look at my profile and setup a call.