#Without root (total memory, available memory, etc)
less /proc/meminfo
vmstat -s
free -mh
#Or you can just look at the 4th and 5th line of "top" command
top
#And press "m" letter from the keyboard while looking at "top" 
#for a better visualization of free RAM

#With root access (Memory type, speed, free, ...) -> more infos
sudo dmidecode -type memory