As mooreds wrote, I'd start with Edgar and the SEC. I'd also check state records (i.e. are they a Delaware C corp? I should be able to pull public information about the filing, same for most states at varying levels of difficulty).
I used to get annoyed at all of the chest-beating press releases about how much money a company had raised until I realized it was a form of external validation. None of the companies which burned me on equity/funding issues wanted to talk publicly about their fundraising rounds, it was always "we're going to a do a press release, it's just not the right time now". If you publicly state "We raise a $2.5MM Series A from the following investors" I can go out and verify that.
Bonus story: a "company" recruited me hard to be their CTO. Some wining and dining. But a lot of handwaving about the business model and where the funding was sourced from. The founders wanted me to bring their "vision" of social e-commerce to market utilising a proprietary algorithm which would be disclosed to me only upon taking the role. When pressed about corporate structure and funding they eventually disclosed that funding was coming in the form of personal checks written to cover bills as they came in, by the domestic partner of one of the founders. No C Corp, not even an LLC. I declined.
I'm curious, how would you go about doing this?