If everybody who created a font followed all the rules and guidelines, the
answer would be easy, because there's an internal flag that says whether or not
a font is monospaced.  Unfortunately, many fonts that are in fact monospaced
don't have that flag set properly.

The free Microsoft font properties extension is very useful, but it doesn't
identify monospaced fonts.

A properties extension that gets installed with "FontExpert" from Proxima
software, DOES show whether or not a font has the monospaced flag set.

Many font utilities, such as Typograf from Neuber, display whether a font is
fixed or variable width.

 - Character