Determine the operating voltage of a SIM card
The ETSI standard ETSI TS 102 221 describes the procedure for switching (and thus finding the appropriate) supply voltage in section 6.2:


  6.2 Supply voltage switching
  The terminal shall initially activate the UICC with the lowest voltage
  class available. If no ATR is received, the UICC shall be deactivated
  and activated with the next higher class, if supported by the
  terminal. If an ATR is received at the first applied voltage class,
  the contents of the ATR shall be analysed by the terminal. If the
  operating class used by the terminal is not supported by the UICC, the
  terminal shall deactivate the UICC and activate it with a supply
  voltage class indicated by the UICC. If the ATR is corrupted, the
  terminal shall perform the procedure at least 3 times using the same
  operating class before rejecting the UICC. In case of 3 consecutive
  corrupted ATRs, the terminal may activate the UICC with the next
  higher class. The terminal is restricted not to use but the next
  higher class in the retrial attempt in this case.