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