How to know which partition of HDD is used by my ubuntu installation ?

I want to know on which partition my Ubuntu is installed. Previously also I had ubuntu installed but it crashed. After I installed new own. Now I am getting confused which is on which partition :(
Hare is o/p of : fdisk -l
 Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x2a429592

   Device Boot Start End Blocks Id System
/dev/sda1 * 63 94260855 47130396+ 7 HPFS/NTFS/exFAT
/dev/sda2 94261246 121661439 13700097 5 Extended
/dev/sda3 121661440 952297471 415318016 7 HPFS/NTFS/exFAT
/dev/sda5 120416256 121661439 622592 82 Linux swap / Solaris
/dev/sda6 94261248 120416255 13077504 83 Linux

Partition table entries are not in disk order

I specifically want to know about it because I want to install third OS on unused partition.

Thanks in advance.
Have a good day !