testing for existence of compilers/executables

[John]

I am working with my build system using scons. I would like
to test the existence of 'doxygen' or any other compiler/executable
in the path (like gcc/icc/...)

What is the most efficient way to find this out using python? using
scons?
Is there a way to list all C/C++/fortran compilers available on a
machine
using python so that I can give my user an option to select one?

Thanks a lot for your help,
--j