You can validate an idea by using the tools from advertising networks to research their userbase. For example, you can use Google's Keyword Planner to see how many people search for specific terms or Facebook's Ad Design to see how many people have a specific interest. Optionally, you can even purchase some ads and guage interest in your idea by seeing how many clicks you get.
You can validate an idea by seeing how big of a community there is for it. How many meetups, conferences, and talks are there? How many people participate? If you go to one of these events, can you get some of the attendees to be interested in your idea?
Google consumer surveyshttp://www.google.com/insights/consumersurveys/home
Imagine surveying the web with a tool so precise it could predict the 2012 presidential election. With Google Consumer Surveys, you choose your target audience, type your question and watch the results roll in within hours.
If there are competitors or similar products, you can research their website traffic and usage to guage the size of the market.
If there are competitors or similar products, you can research their mobile traffic and usage to guage the size of the market.
Market Research and Reports
Startup Ideas and Validationhttp://www.ybrikman.com/writing/2016/02/14/startup-ideas-and-validation/
Every startup begins with an idea. This is a talk that discusses how to come up with startup ideas and validation techniques you can use to pick the ideas worth working on.