Sometimes it can be very difficult or impossible to find it out, because some webmasters try to hide the sourced but most of the time it is pretty easy to find this information. The simple steps are as follow: Make sure to play the video and wait after the preload ads end and then right click near the video player area and select “Inspect” to open Developer Tool page and then try to find the tag reading something like: < div class = " video-player "> and somewhere there you should find the video source link.
youtube.com/watch?v=zWRAXPJdsEY
youtube.com/watch?v=TXa-lAD22wQ
youtube.com/watch?v=waE3J0Jej_0
youtube.com/watch?v=ZqxPZeznU_Y
If it is not clear enough, check out these videos and they follow pretty much the same process.