what is the best way to determine system OS?

[googleboy]

Hi there.

I am writing a little app tha tI would like to make cross-platform
(debian, RH, Fedora, Solaris, AIX, etc)

Originally I decided to check what uname returned, as I didn't think it
mattered beyond the detail of Linux or SunOS etc.

Recently I have learned that FC3 breaks my script,  so I need to be
able to determine not simply "Linux" but to know exactly what unix the
script is being run on.

What is the best way to determine this?

Thanks!

googleboy