I think your question is related to your other
post? If your SD card already has data on it, it probably already has a filesytem on it because you need a file system in order to write data to it. If it's a sd card it'll probably have FAT32 filesystem on it and it, probably the whole sd card is formatted with that filesystem. Just put the sd card reader of your laptop/desktop and you can just do same as we did before.
Then you can see the size of the partition on your sd card. Then substract size of your partition of the size of your sd card and that's how much space you will have left over. If you end up with 0, then all the space of the sd card will have been formatted with a filesystem. You also use the following command to see if there are any partitions on it
or
Then you can see the disks and the partitions you have. And you can do it via a graphical interface, which you have to install first
Code:
sudo apt-get install gparted
or
# apt-get install gparted
Then somewhere under administrative tools you can open gparted and see your disks/partitions via a graphical interface. But to get back to your question, you need to be sure that the current filesystem isn't taken up all of the space on your sd card. If all space of the sd card is formated with a filesystem then you can't install something next to it. If only a small portion is formated with a filesystem, like for example 2GB(of32GB), then you can use the free space to create another file system via the ubuntu installer/disk partitioner. Which you can then install another operation system on. Does that make sense?