I've wondered this too. I think I know how the bricking is done. I think one of the blocks (63?) gives the file full access of the file system. doing this manually seems like a lot of work. I don't own a hacked switch, but I've read that tinfoil (is that it?) can determine if it's been tampered with. what I usually do is just verify with nsp verify on the pc. not really sure, but it may be able to tell if the file has been tampered with.
basically only official nca will have signature verified. so homebrew nsps and converted (xci to nsp and vice versa) will not pass, but that doesn't necessarily mean it has brick code in it
afaik dlc only has data nca, no executable in it so there's no brick code
and as for dlc with signature not passed, usually they are not the actual dlc but simply unlocker