in linux, you can use
cat /proc/meminfo to see total swap, and free swap (all linux)
cat /proc/swaps to see which swap devices are being used (all linux)
swapon -s to see swap devices and sizes (where swapon is installed)
vmstat for current virtual memory statistics
in Mac OS X, you can use
vm_stat to see information about virtual memory (swap)
ls -lh /private/var/vm/swapfile* to see how many swap files are being used.
in Solaris, you can use
swap -l to see swap devices/files, and their sizes
swap -s to see total swap size, used & free
vmstat to see virtual memory statistics