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
In which, ‹label-name› is exactly what you would use in a
\labelcommand, 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
And of course, remember you’re dealing in internal commands, and pay attention to the at-signs.
All the above can be avoided by using the labelcas package: it provides commands that enable you to switch according to the state of a single label, or the states of a list of labels. The package’s definition is a bit complicated, but the package itself is pretty powerful.