You can't tell from just looking at the console, MCU bricks are caused by internal hardware damage. So even if the NAND chip was fried, unless it combusts (which isn't likely) you won't even seen scorching on the motherboard. The seller would have to confirm the type of brick for you, if they don't know then it's a gamble.
I can say through observation that more often than not when people suspect an MCU brick, it's almost always a broken/detached ribbon cable. Anything controlled by the MCU SoC can cause the console to boot into a blackscreen; those include the home button, LED, Camera and a few other peripherals. In the "assumed" bricks I have seen here, it was always the broken ribbon cable for the camera that was causing a "brick."
Basically, inspect the innards of the console for anything out of place prior to assuming it's a software related brick because software MCU bricks are impossible. If it's software related, it can at least be booted with NTRBootHaX.
From what I heard MCU bricks were patched in 3DS firmware after 7.x or so, so there should be no way for them to happen on a n2DS XL.
That said buying broken consoles is kind of hit and miss unless the problem is obvious. A BSoD brick could be a broken NAND chip which you won't be able to fix. If you're lucky it's an easy fix with ntrboot. Hard to say without buying it though.