Mixin generates an SMAP in the target class when transforming it, which allows you to obtain which mixin added a particular line of code. Or perhaps it might be easier to use @MixinMerged.

@Earthcomputer Where can I find more information on this 'SMAP'?

https://github.com/SpongePowered/Mixin/blob/b1cd46c8c4960eae026bcd6dd9bafb59cafa4d52/src/main/java/org/spongepowered/asm/mixin/struct/SourceMap.java
https://jcp.org/aboutJava/communityprocess/review/jsr045/

In what way should it be displayed? Added to stack traces like Forge? Taken into account when determining suspected mods?

I'd just replace the source file

For what it's worth I did this a few months ago. Ran happily enough alongside NEC back then and I would presume it still does now.

@Chocohead Hey, I would like to use the code you posted, but it does not have a license. Do you permit me using that code for Not Enough Crashes?