You can do this by running the 'free -m' command, a sample output is below:

free -m
total used free shared buffers cached
Mem: 512 136 375 0 0 135
-/+ buffers/cache: 1 510
Swap: 2048 0 2048

Now don't worry if you think the amount used is high linux is very good at caching stuff to memory and will
free it up instantly should programs need it to run.

A great explanation is available on linuxatemyram.com

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