How do we find out which Corporations with offshore headquarters..

got government contracts? This is a good campaign issue.

If you have a stock broker, ask him/her.

... aren't just one corporation; they're many interlinked (often wholly-owned) corporations, some set up solely to take advantage of offshore tax shelters, some set up to erect liability "firewalls" to protect other (limited liability) sibling corporations, and some set up to take advantage of lax (accounting, labor, environmental, etc.) regulations.

Enron was literally hundreds of corporations, all operating either wholly or mostly under the "Enron" umbrella.

This is part of the cancer of corporatism. While they're (wrongfully) treated like "people" under many laws, they get to own other "people". They are both property and "people" under the law. Somewhat like the "living dead," they're immortal and cannot be locked up in a jail cell.