It's hard to check your theme in all browsers. There are a number of tools which are available to help look at your theme in multiple browsers.
- Browser Shots is freely available but can take a while to get your screenshots.
- BrowserCam is paid service with a 24 hour trial.
On Windows you can use Internet Explorer and download Firefox or Opera. On Linux you can use Konqueror, a KHTML based browser, which Safari uses on MacOS, Opera, Firefox for Linux, and Internet Explorer can be run under WINE. On Mac OSX you can use Safari and download Firefox or Opera.