How can I tell when a string is in fact a number?

[Roy Smith]

"Alex Martelli" <aleaxit at yahoo.com> wrote:
> I dread the task of explaining to a beginner
> why [...] why 3+2j 'is a number'

Heck, you're going to have enough trouble explaining to some people why 
3+2j is a number but 3+2i isn't :-)