How do I interpret electrical system indications in a G1000 C-182?
You can find this in the C182T Operating Handbook. Section 3 has information about what values should be considered a malfunction:


  HIGH VOLTS ANNUNCIATOR COMES ON OR M BAT AMPS MORE THAN 40


It also says that the M bus should be at least 20 volts in normal operation:


  The Main Battery supplies electrical power to the Main and Essential
  Buses until M BUS VOLTS decreases below 20 volts. When M BUS VOLTS
  falls below 20 volts, the Standby Battery System will automatically
  supply electrical power to the Essential Bus for at least 30 minutes.


Section 4 has the expected values when starting the engine:


  
  BUS E Volts
  
  CHECK (Verify 24 VOLTS minimum shown)
  
  M BUS Volts
  
  CHECK (Verify
  1.5 VOLTS or less shown)
  
  BATT S Amps
  
  CHECK (Verify discharge shown (negative)
  
  


Section 7-54 describes the electrical system in detail, including wiring diagrams. It says:


  Normal bus voltages with the alternator operating shall be about 28
  volts.


And:


  Indicated voltages between
  24.5 and 28 volts may occur during low engine RPM conditions


As for the ammeter:


  A positive current value (shown in white) indicates that the battery
  is charging. A negative current value (shown in amber) indicates that
  the battery is discharging. In the event the alternator is not
  functioning or the electrical load exceeds the output of the
  alternator, the main battery ammeter indicates the main battery
  discharge rate. In the event that Standby battery discharge is
  required, normal steady state discharge should be less than 4 amps.


So long story short, all the information you need is in the POH and the best way to get all the details is to read it carefully. Your POH may be slightly different from the one I've quoted here, so if there are any differences then you should follow it, not this answer.