Nsis Decompiler
Generally speaking, you can either detect when the data is read from the file (by monitoring functions as ReadFile), or when it's being decompressed (by monitoring the decompression function once you find it). Ken bendat. In some cases the program may read the compressed data directly and pass it to the decompression function, but in others it could read some chunk of file together with the header/metadata, parse that header, and then pass only the trailing compressed data to the decompressor.
Nsis Decompiler
Universal Cloud Decompiler. Reverzor can decompile and recover the original NSIS script inside the NSIS setup file, allowing you to recompile the NSIS project. But Ippi, do you understand that even if there would be some kind of 'universal NSIS decompiler', it wouldn't make any difference.
There is no single 'right way' to figure out where exactly the compressed data is in the file. That said, NSIS is open-source and you can just look to see how it works. There are also to extract files from NSIS installers.