Here you go. Every Model A owner should have at least a basic analog DC volt meter, and they don't come much cheaper than this. If you have a bad connection in the charging circuit, or if the battery is too weak or small, then the generator will put out too high a voltage, unless you use an electronic voltage regulator, as I do.
http://www.ebay.com/itm/1pcs-New-DC...773356?hash=item27ee7b28ec:g:8CMAAOxy2BBSUjHt