Bad Backlight - Display or Logic?

Hi - I have an A1502 with a bad backlight (flashlight test shows the display is working.) How can I diagnose whether the problem is with the logic board or the display before purchasing and swapping in either?

Extra detail: the backlight did not completely die all at once. It stopped working, then came back on a few days later, went out again, and repeated this cycle for a week or two before finally turning off for good.

Here are some pics - and the backlight is currently back on.