This post will show how a LCD 20×4 can be tested with Arduino. Things you need :
Step 1 : Set your board like this, make sure where is 16th and 1st pin on LCD.
Step 2 : Provide power to the breadboard. To the upper line connect +5V from Arduino Mega 2560 and to lower line connect GND from Arduino Mega 2560. I was out of proper colored wires so I used Green for ground and Yellow for +5V.
Step 3 : Connect the back light for the LCD. Connect pin 16th of LCD to GND on breadboard and pin 15th of LCD to +5V of breadboard.
Step 4 : Plug in the USB to Serial cable from Laptop to Arduino Mega 2560.
Step 5 : As soon as you plug in the cable Arduino gets powered up and LCD lights up. If it isn’t working the check for loose connections and pin connections.