There is a simple way to check if a filament is open or not.  Apply a
low current square wave at the switch side of the lamp. It doesn't
matter if the other side of the lamp  goes to +12Vdc or Ground, since
one polarity of the square wave signal will try to lit the lamp, and the
current is an indication of good filament.    It is not so common to
check lamp filament when the lamp is not lited, but it can be done.

Just a curiosity about lamp filaments:
Airplane crash investigation take a special attention to panel lamp
filaments.  They can almost identify if a lamp was on or off at the
crash moment.  Alarm and "red lamps" are important to understand why an
airplane crashed.  A lited lamp has a hot filament that breaks it easily
at a crash impact. A cold filament is more difficult to break... well...
it helps.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm