Auto-detect flash environment (browser or AIR)

I’d like to use the same code base for both desktop AIR and browser plugin applications, with some changes in the manner in which data is loaded and saved. Ideally, I would like to auto-detect the current environment so I know where to look for the data to be loaded, either from the filesystem or from a URL. Is there a simple test that can accomplish this without relying on a hack? Thanks in advance.