Corporate foundations must file 990-PFs where they record their grants. But if a corporation makes its charitable contributions directly from the company itself, a listing of its grants may not be so easy to find.
Some corporations have reports about their corporate contributions or community relations activities on their websites. Some of the larger corporate giving programs are listed in grants databases. (Note that the Foundation Center's Corporate Giving Online is a subset of the same funders contained in the Foundation Directory Online.)
But the easiest way to find corporate donations is to go to NOZA. It's very reasonably priced. Simply enter the name of the company, and you will get a list of its grants and contributions listed on the web in nonprofits' annual reports, press releases, and other publications.
You can also try Googling the corporation, linking the company name with "gift," "grant," "donation," etc. Try Google News Archives for past donations.