To check your memory information, just enter this command on the terminal

cat /proc/meminfo

And as its name says, it will display the memory information (mem info).

This is the output of the command in my PC, you do not need to be root to run it.

MemTotal:      2063748 kB
MemFree:       1745124 kB
Buffers:         19936 kB
Cached:         169268 kB
SwapCached:          0 kB
Active:         118432 kB
Inactive:       155208 kB
HighTotal:     1169640 kB
HighFree:       905780 kB
LowTotal:       894108 kB
LowFree:        839344 kB
SwapTotal:     2650684 kB
SwapFree:      2650684 kB
Dirty:            3064 kB
Writeback:           0 kB
AnonPages:       84436 kB
Mapped:          34700 kB
Slab:            23900 kB
SReclaimable:    17164 kB
SUnreclaim:       6736 kB
PageTables:        560 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:   3682556 kB
Committed_AS:   301852 kB
VmallocTotal:   114680 kB
VmallocUsed:     48188 kB
VmallocChunk:    65644 kB
HugePages_Total:     0
HugePages_Free:      0
HugePages_Rsvd:      0
HugePages_Surp:      0
Hugepagesize:     4096 kB

Other applications to check memory are:

Conky, which also shows you other valuable information.

htop