An initialised swap space is taken into use with swapon. This command tells the kernel that the swap space can be used. The path to the swap space is given as the argument, so to start swapping on a temporary swap file one might use the following command.
$ swapon /extra-swap $
/dev/hda8 none swap sw 0 0 /swapfile none swap sw 0 0
You can monitor the use of swap spaces with free. It will tell the total amount of swap space used.
$ free total used free shared buffers Mem: 15152 14896 256 12404 2528 -/+ buffers: 12368 2784 Swap: 32452 6684 25768 $
That last line (Swap:) shows similar information for the swap spaces. If this line is all zeroes, your swap space is not activated.
The same information is available via top, or using the proc filesystem in file /proc/meminfo. It is currently difficult to get information on the use of a specific swap space.
A swap space can be removed from use with swapoff. It is usually not necessary to do it, except for temporary swap spaces. Any pages in use in the swap space are swapped in first; if there is not sufficient physical memory to hold them, they will then be swapped out (to some other swap space). If there is not enough virtual memory to hold all of the pages Linux will start to thrash; after a long while it should recover, but meanwhile the system is unusable. You should check (e.g., with free) that there is enough free memory before removing a swap space from use.
All the swap spaces that are used automatically with swapon -a can be removed from use with swapoff -a; it looks at the file /etc/fstab to find what to remove. Any manually used swap spaces will remain in use.
Sometimes a lot of swap space can be in use even though there is a lot of free physical memory. This can happen for instance if at one point there is need to swap, but later a big process that occupied much of the physical memory terminates and frees the memory. The swapped-out data is not automatically swapped in until it is needed, so the physical memory may remain free for a long time. There is no need to worry about this, but it can be comforting to know what is happening.