i know there are apps on PS that can reveal previously saved passwords but here is the actual path
using ES file manager (must be rooted obviously)
Go to: Device/Data/Misc/Wifi
hapo chini utaona a file named wpa_supplicant.conf
Open the file and make sure you use ES File Explorer ‘s built-in text/HTML viewer for the task.
Now in this file you will be able to see the network SSID and their passwords next to it.