Checking var is a number?

[dav.phillips at ntlworld.com]

Hi,
I am very new to all this and need to know how to check
a variable to see if it is a number or not. Also can anyone
recommend a reference book apart from dive into python
preferably a reference with good examples of how to impliment
code.

The project i have been given to work in is all CGI written
in Python.

Many Thanks 
David Phillips