How do I know which Python install is being used? If I type the command which python, then I get:

Oliviers-iMac:~$ which -a python
/System/Library/Frameworks/Python.framework/Versions/2.7/bin/python
/usr/bin/python


If I then type python --version, how do I know which Python install I'm getting the version of? Are the two locations above 2 different Python installs?
/System/Library/Frameworks/Python.framework/Versions/2.7/bin/python is most likely a symlink. 

readlink on a simlink will give you the target. 

For example on my Mavericks installation I have following : 

$readlink /System/Library/Frameworks/Python.framework/Versions/2.7/bin/python
python2
$readlink /System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2
python2.7


Which means when I run python it actually runs /System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7



Also there is something funny, when running /System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7

Try this

&gt;&gt;&gt; import sys
&gt;&gt;&gt; print sys.executable 


Which will return this : 

/System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOs/Python