How to know if a system command doesn't exist?

[Nicolas Fleury]

Hi,
	How can I know if a system command doesn't exist?  All the ways I have 
tried behave like if the system command was existing but returning one. 
  I don't want to sound provocative, but open in Perl returns an error 
in a command doesn't exist, so I'm sure we could have an exception in 
the same case for popen in Python?

Thx and Regards,

Nicolas