The token \jobname amusingly produces a sequence of characters
whose category code is 12 ('other'), regardless of what the characters
actually are. Since one inevitably has to compare a macro with the
contents of another macro (using \ifx, somewhere) one needs to
create a macro whose expansion looks the same as the expansion of
\jobname. We find we can do this with \meaning, if we strip
the "\show command" prefix.
The full command looks like:
\def\jobis#1{FF\fi
\def\predicate{#1}%
\edef\predicate{\expandafter\StripPrefix\meaning\predicate}%
\edef\job{\jobname}%
\ifx\job\predicate
}
And it's used as:
\if\jobis{mainfile}%
\message{YES}%
\else
\message{NO}%
\fi
Note that the command \StripPrefix need not be defined if you're
using LaTeX - there's already an
internal command
\strip@prefix that you can use.