[Vwdiesel] cold start issues

[Kneale Brownson]

The 94 Jetta TD doesn't want to start when cold.  I have power to the glow plugs, I get fuel, the battery shows 12.4 volts static.  If ambient temperature gets somewhere above about 60 degrees (F), it'll start readily.  Just plain won't catch when colder if it's sat for more than six or eight hours.  
   
  Could the glow plugs not function despite power to their wiring?  How would I test those?