how to test cross browser testing in QTP?

You will need to create separate Actions which take care of different OS and Browsers

Cross Platform Testing:

Using the Built in Environment Variable you can dig up the OS information.
Eg. Platform = Environment("OS"). Then based on the Platform you need to call the actions which you recorded on that particular platform.

Cross Browser Testing:

Using this code Eg. Browser("Core Values").GetROProperty("version") you can extract the Browser and its corresponding version. Ex: Internet Explorer 6 or Netscape 5. Based on this value you call the actions which are relevant to that browser.