When voltages are given on a circuit diagram these are referenced to the common rail or "circuit ground". This is where you should put the black lead of your meter when making measurements.
Often the common rail will be connected to the chassis (but it doesn't need to be), and sometimes the common rail will also be connected to the earth safety ground (e.g. for noise rejection and signal quality), but it doesn't have to be.
Typically, anywhere on the schematic that shows the chassis ground symbol will be a suitable place to put the black meter lead.