How to find out which partition is Ubuntu installed on?

In a terminal, run the command df and look at the output. The column headed "Mounted On" will have a "/" against the partition that holds your root filesystem. This is where your system is installed.

Ok so if you're running Ubuntu 12.04 you should have GParted (a partitioning tool) installed, to find it click on the Ubuntu launcher icon and type GParted and it should show up like this:

Click on it and then you should get something like this:

Your Ubuntu partition will be on the one which has / in the mount point column. Windows usually takes primary partitions so Ubuntu is not likely to be /dev/sda1 or /dev/sda2, but feel free to post a screenshot of what your GParted shows if you need more help. Ubuntu is usually installed on ext File Systems as shown as an example in my screenshot above.