How to tell which local branch is tracking which remote branch in Git?

Using the example of my copy of Puppet checked out from the upstream Git repository on Github.com...

$ git remote show origin
* remote origin
  Fetch URL: git://github.com/reductivelabs/puppet.git
  Push  URL: git://github.com/reductivelabs/puppet.git
  HEAD branch: master
  Remote branches:
    0.24.x                 tracked
    0.25.x                 tracked
    2.6.x                  tracked
    master                 tracked
    next                   tracked
    primordial-ooze        tracked
    reins-on-a-horse       tracked
    testing                tracked
    testing-17-march       tracked
    testing-18-march       tracked
    testing-2-april        tracked
    testing-2-april-midday tracked
    testing-20-march       tracked
    testing-21-march       tracked
    testing-24-march       tracked
    testing-26-march       tracked
    testing-29-march       tracked
    testing-31-march       tracked
    testing-5-april        tracked
    testing-9-april        tracked
    testing4268            tracked
  Local branch configured for 'git pull':
    master merges with remote master
  Local ref configured for 'git push':
    master pushes to master (up to date)

Then if I were to execute the following:

$ git checkout -b local_2.6 -t origin/2.6.x 
Branch local_2.6 set up to track remote branch 2.6.x from origin.
Switched to a new branch 'local_2.6'

And finally re-run the git remote show origin command again I will then see the following down near the bottom:

  Local branches configured for 'git pull':
    local_2.6 merges with remote 2.6.x
    master    merges with remote master

For all branches:

git branch -avv

For local branches only:

git branch -lvv

For remote branches only:

git branch -rvv

shows you all branches as well as the name of the upstream branch.

Jeremy Bouse illustrates how git remote show displays tracking information. That should be sufficient if you only want the information for human consumption.

If you plan on using the information in an automated context (e.g. a script) you should use the lower-level (“plumbing”) git for-each-ref instead.

% git remote show origin
* remote origin
⋮
  Local branches configured for 'git pull':
    master merges with remote master
    pu     merges with remote pu
⋮
% git for-each-ref --format='%(refname:short) <- %(upstream:short)' refs/heads
master <- origin/master
pu <- origin/pu

The git for-each-ref learned the %(upstream) token in Git 1.6.3. With earlier versions of Git you will have to extract the tracking information with git config branch.<name>.remote and git config branch.<name>.merge (probably using git for-each-ref to build the commands for each local branch name).