Run this command from a terminal window. This works on debian, ubuntu, and redhat. I'm sure it works on others but those are the only ones I have access to.

cat /proc/meminfo

You should see results similar to this:

MemTotal:      3615716 kB
MemFree:        132528 kB
MemShared:           0 kB
Buffers:         50028 kB
Cached:        1572772 kB
SwapCached:      80716 kB
Active:         580208 kB
Inactive:      1142848 kB
HighTotal:     2752384 kB
HighFree:         8576 kB
LowTotal:       863332 kB
LowFree:        123952 kB
SwapTotal:     2104504 kB
SwapFree:      1064540 kB