DomainTools.com has a service where you can order a report and find out all the domains a company owns. But if the domains are registered under different names as sometimes happens, the report may be incomplete. Also the reports can be somewhat expensive.
The other option is that some web sites have a function that will show you all of the domains on a name server or web server. If you can find one of the domains a company owns, sometimes you can see all of their domains if they have them hosted or parked on the same server. Still, the list you get may be incomplete.