I did a little research on VDO tachs and it looks like the wire coming up from the alternator is going to be a 12 volt square wave. That's going to be hard to measure with out and oscilloscope but you can try it with a regular volt meter. Set for AC voltage and measure between the G terminal on the tach and the negative. If you get something like 1.5 to 5 volts AC, I would say the signal is good.

One last thing to check. Make sure the alternator D terminal is well connected to the battery negative.

Mike