How to find the remaining fuel without electrical power on the MD-80?
The normal Digital Fuel Quantity Display in the flight deck does not work without AC power, meaning you need to have either the APU running or external AC power connected:


  Fuel Tank Quantity Indicating
  
  A standardized modular fuel gauging system is used which requires no
  adjustment or calibration after system installation or component replacement.
  
  The Digital Fuel Quantity Display is located on the center instrument panel
  to indicate the quantity of the three tanks and gross weight. Operation of this
  display requires normal AC power.


(MD-80 FCOM Sec. 13 - Fuel)

The Fuel Load Display Panel on the right wing however can be powered via the battery:


  Fueling POWER Switch
  
  OFF - Removes power from fuel load selector display panel and fuel fill valve switches.
  
  ON - Supplies power from battery or external power to fuel load selector display panel and fuel fill valve switches.


(MD-80 FCOM Sec. 13 - Fuel - Controls and Indicators)

You can read off the current fuel levels in the upper row of the display panel:



(image source: MD-80 FCOM Sec. 13 - Fuel - Controls and Indicators)

Without battery power, manual measurements of fuel tank quantities are still possible with a magnastick:


  Fueling and Defueling
  
  [...]
  
  All fueling operations can be accomplished with airplane or externally
  supplied electrical power. If electrical power is not available for fueling
  operations, all fuel tank valves can be manually operated and fuel quantity
  measured using magnasticks.


(MD-80 FCOM Sec. 13 - Fuel)

The following section from a Dash-8 manual describes how the magnastick (or magnetic dipstick) works:


(image source: Dash-8 Q400 FCOM)


  
  Magnetic Dipstick Magnet
  Float Magnet
  Fuel Level in Main Tank
  Calibrated Magnetic Dipstick
  
  
  the magnetic dipsticks give an alternate means to measure the fuel quantity when the aircraft is on the ground
  the magnetic dipstick is accessed at the bottom of the wing
  when the rod is released, it moves slowly down until the rod magnet is attracted by the float magnet
  
  


(Dash-8 Q400 FCOM - Fuel)


  Magnasticks
  
  Two magnetic dipsticks (magna sticks) in each tank provide an alternate means of measuring fuel quantity when on the ground. Each consists of a calibrated rod, which slides within a tube extending vertically from the bottom of the fuel tank. When the rod is released, allow it to fall slowly until resistance is felt as the rod magnet becomes attracted to the float magnet. Read the tank contents from the bottom of the rod in U.S. gallons or in litres.


(Dash-8-200-300 FCOM - Fuel)