How to determine compression method of a ZIP/RAR file

For ZIP - yes, zipinfo

For RAR, the headers are easily found with either 7Zip or WinRAR, read the attached documentation

Via 7-Zip (or p7zip) command line:

7z l -slt archive.file

If looking specifically for the compression method:

7z l -slt archive.file | grep -e '^---' -e '^Path =' -e '^Method ='

This is a fairly old question, but I wanted to throw in my two cents anyway since some of the methods above weren't as easy for me to use.

You can also determine this with 7-Zip. After opening the archive there is a column for method of compression:

I suggest hachoir-wx to have a look at these files. How to install a Python package or you can try ActivePython with PyPM when using Windows. When you have the necessary hachoir packages installed, you can do something like this to run the GUI:

python C:\Python27\Scripts\hachoir-wx

It enables you to browse through the data fields of RAR and ZIP files. See this screenshot for an example.

For RAR files, have a look at the technote.txt file that is in the WinRAR installation directory. This gives detailed information of the RAR specification. You will probably be interested in these:

 HEAD_FLAGS      Bit flags: 2 bytes
                 0x10 - information from previous files is used (solid flag)
                 bits 7 6 5 (for RAR 2.0 and later)
                      0 0 0    - dictionary size   64 KB
                      0 0 1    - dictionary size  128 KB
                      0 1 0    - dictionary size  256 KB
                      0 1 1    - dictionary size  512 KB
                      1 0 0    - dictionary size 1024 KB
                      1 0 1    - dictionary size 2048 KB
                      1 1 0    - dictionary size 4096 KB
                      1 1 1    - file is directory

Dictionary size can be found in the WinRAR GUI too.

 METHOD          Packing method 1 byte
                 0x30 - storing
                 0x31 - fastest compression
                 0x32 - fast compression
                 0x33 - normal compression
                 0x34 - good compression
                 0x35 - best compression

And Wikipedia also knows this:

The RAR compression utility is proprietary, with a closed algorithm. RAR is owned by Alexander L. Roshal, the elder brother of Eugene Roshal. Version 3 of RAR is based on Lempel-Ziv (LZSS) and prediction by partial matching (PPM) compression, specifically the PPMd implementation of PPMII by Dmitry Shkarin.

For ZIP files I would start by having a look at the specifications and the ZIP Wikipedia page. These are probably interesting:

  general purpose bit flag: (2 bytes)
  compression method: (2 bytes)