I currently work at a multi-national entity overseas, and I see them using an internally-developed tool which could be used by bigger businesses in the US. Now how can I validate my idea?
Is it your idea?You just said you "see them using..."
Anyway, to validate an idea for the US Market, you need an MVP and a promising team if you want to go for Y Combinator. Yet, a team full of internationals would have a low chance of getting in.
Kickstarter if it's a physical product, Google Adwords/Facebook Ads geared towards US geo, Appsfunder if it's an app, and getting the phone to speaking with potential customers.