On all / most Linux based systems with the /proc/ filesystem there are a set of system information variables. If you execute a cat /proc/meminfo on your system you should see an output of something along the lines of:

erik@debian:~$cat /proc/meminfo 
MemTotal:        2960104 kB
MemFree:          650748 kB
Buffers:           24076 kB
Cached:           337756 kB
SwapCached:       179952 kB
Active:          1311468 kB
Inactive:         680516 kB
Active(anon):    1197760 kB
Inactive(anon):   458340 kB
Active(file):     113708 kB
Inactive(file):   222176 kB
Unevictable:          32 kB
Mlocked:              32 kB
SwapTotal:       2931820 kB
SwapFree:        1978820 kB
Dirty:              1736 kB
Writeback:             0 kB
AnonPages:       1563940 kB
Mapped:           164036 kB
Shmem:             25968 kB
Slab:              64828 kB
SReclaimable:      37260 kB
SUnreclaim:        27568 kB
KernelStack:        3072 kB
PageTables:        38040 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     4411872 kB
Committed_AS:    3877408 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      184260 kB
VmallocChunk:   34359534964 kB
HardwareCorrupted:     0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:     2866112 kB
DirectMap2M:      147456 kB

This output displays the current systems total memory, the amount of free memory, how much memory is cached among other things. It is a quick way to check our systems memory data.