Suppose you need to know that the argument of your command is empty:
that is, to distinguish between \cmd{}
and \cmd{blah}. This is pretty simple:
\def\cmd#1{%
\def\tempa{}%
\def\tempb{#1}%
\ifx\tempa\tempb
<empty case>
\else
<non-empty case>
\fi
}
The case where you want to ignore an argument that consists of nothing
but spaces, rather than something completely empty, is more tricky.
It's solved in the code fragment ifmtarg, which defines
commands \@ifmtarg and \@ifnotmtarg, which distinguish (in
opposite directions) between a second and third argument. The
package's code also appears in the LaTeX memoir class.
Ifmtarg makes challenging reading; there's also a discussion of the issue in number two of the "around the bend" articles by the late lamented Mike Downes.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=empty