How much memory is free?
A common way...
$ free -m total used free shared buff/cache available Mem: 488 228 23 11 236 217 Swap: 0 0 0
The figure under "available" is how much is "Free and available"
i.e. what humans usually mean when they say "free"
To watch it as it changes...
$ watch -n 5 free -m
This will refresh every 5 seconds.
$ cat /proc/meminfo | grep MemAvailable MemAvailable: 223336 kB
$ watch -n 5 'cat /proc/meminfo | grep MemAvailable'
To see a lot of specific info about your machine...
$ sudo lshw
* [Linux Ate My Ram](http://www.linuxatemyram.com/)