Voltmeter placement

By DIY, do you mean you built it yourself, or you installed it yourself? 

Some commercial units come with charge controllers which have voltage compensation, i.e they step up the voltage from what is actually produced in order to make it productive in lower wind speeds.  My Bergey XL.1 does this.  When the wind speed is low, the green LED blinks, indicating voltage compensation.  When the wind is strong enough, the green LED is solid, indicating sufficient voltage.  So in that case, if you wanted to check the output voltage, you'd have to know whether you want the raw output voltage or the battery charging voltage.

Since you mention a diode, I assume that this is a home-built turbine and the diode is used to convert AC from the stator to direct current for battery charging.  In that case, then if you want the raw pre-diode voltage, you need to use an AC voltmeter.  To measure the battery charging DC output, then you would use a DC voltmeter on the battery side of the diode.  BTW, a bridge rectifier would be better than a diode for converting to DC.  With a diode, you only capture half of the power (the positive half of the AC wave), whereas a rectifier captures the full output.