See if you get any readings from L-G with a "Wiggy".
If Ungrounded, you should get a transient "pulse" reading on the Solenoid and Neon Lamp of the wiggy, at the moment the probes make contact - and meter is placed between Line and Ground (the Enclosure of the Switchgear will be fine for the "Ground" side).
The wiggy should react like a Capacitor is discharging rapidly through it.
Only a very quick ( < 100 ms) reaction will be observed, which decays rapidly to a "Zero" reading on the Wiggy.
If you are able to sustain a low level reading, then you may be looking at an Impedance Grounded System.
If the Wiggy holds nice and steady - similar to L-L connections, then it may be Impedance Grounded or Solidly Grounded system.
*** Disclaimer: The items above, are simply examples of some "Common" occurences which I have come across over the Years.
These examples are definitely NOT
100% Fool-Proof, Absolute, Written In Stone or anyway Default to all scenarios!!!
The information is only suggestions to consider!!! Please take all precautionary measures when investigating this system!
In all cases, please take Bjarney's post as sound advice - do not under think the resultant Voltage Readings from Meters on Ungrounded systems!!!
The most effective methods to determine what's what are Visual - Non Contact, determination of the System's Connections schemes (at the Transformer and the Service Equipment), usage of - and understanding of correct Metering Devices, and the most effective - verification via the PoCo.
If important enough, ask the PoCo Design Engineering staff, or possibly the Building Department, for existing Service information.
They normally help out whenever possible, but don't expect instant results / answers when the Offices are swamped and paperwork is stacked 4 feet high.
Lastly, if I sounded coarse in this message, I appologize. I am not trying to sound like a Deee-Oooccck, just making it well known to other readers there are extreme hazards involved, and danger silently lurks behind the deadfront sections of these systems' panelboards and gear sections - waiting for the chance to ... well, for lack of a better term:Fry Up Some Huevos Rancheros!
and the "Huevos" being attached to some person's midsection!