How do you find out if a corporation has a valid 501(c)(3) status? Is there a government site?

Yes, you can refer to "Publication 78" of the IRS, linked below.

How do you find out if a corporation has a valid 501(c)(3) status? Is there a government site?
A 501(c)3 organization means you can give a tax deductible donation. If a company is requesting a donation, ask for proof. I used to work at a foundation that distributed grants. I always required a copy of the 501(c) 3 IRS designation letter.