You would get the service manual for that model and then follow it's instructions on what voltages to measure and then replace the board it calls out. It could be other than these two boards. I'm sure techs have found shorts on other areas but if you are guessing the advice is "replace the cheapest part first."
Service manuals for a lot of the US models is over at samsungparts.com
I have a Samsung model#: un55ju6400fxza. Version#: fd05. When plugged in I get a red power light, but no picture and an electrical buzz from the rear. I opened the back and located the buzz coming from the power supply board, specifically from the silver heat sink looking block on the top left. When I disconnect the ribbon cable to the main board, buzzing stops and backlight/screen comes on. Is this issue with my mainboard or power supply board? I see no swollen capacitors or signs of issues. Thanks in advance for your advice.