Hi I have a Uno 60D 1994
The preheater light is not coming on so it is hard starting the engine It starts ok when warmed up How do I carry out testing the system

No idea, but it sounds like the glow plugs are worn. About all you can do as far as I know is substitute them with new ones.

Good luck!

When you turn the igntion on, there should be over 12v going to the glow plugs, put the red lead of a voltage meter to the nut on the end of the glow plug and the black to the body of the car or the negative of the battery, you should have voltage for around 12 seconds and about 20 seconds after the engine is started.
If its a mk2, try looking behind the speaker grill in the drivers footwell, get your head under the dash and there should be a controller box for the glow plugs timer box, ive knocked the plug off the bottom before and had troubles starting!!