I've always tested them out of the head with a spare battery or booster pack - as mentioned a solid 12v supply and they should glow bright red/orange within 2 or 3 seconds and longer and they are getting lazy.

However, its been suggested already modern oil burners dont really need them yet - IIRC my golf doesn't heat the plugs unless it 5'c or lower...

Is it the 90hp (88?) 1.8 tdi as my friends Connect had this lump and suffered starting issues - in the end the starter was changed for the 3rd time and it was as good as gold until the big ends went @ -120k :S

Tom