People seem to want to know (at run time) if a label is undefined (I don't actually understand why, particularly: it's a transient state, and LaTeX deals with it quite well).
A resolved label is simply a command:
\r@<label-name>; determining if the label is set is
then simply a matter of detecting if the command exists. The usual
LaTeX internal way of doing this is to use the command
\@ifundefined:
In which, <lab-name> is exactly what you would use in a\@ifundefined{r@lab-name}{undef-cmds}{def-cmds}
\label command, and the remaining two arguments are command
sequences to be used if the label is undefined
(<undef-cmds>) or if it is defined
(def-cmds).
Note that any command that incorporates \@ifundefined is naturally
fragile, so remember to create it with \DeclareRobustCommand or to
use it with \protect in a moving argument.
If you're into this game, you may well not care about LaTeX's
warning about undefined labels at the end of the document; however,
if you are, include the command \G@refundefinedtrue in
<undef-cmds>.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=labundef