Выполните в консоли команду.
user@home:~$ free -m
total used free shared buffers cached
Mem: 3920 2669 1250 0 76 1226
-/+ buffers/cache: 1367 2553
Swap: 4093 0 4093
ИЛИ
# cat /proc/meminfo
MemTotal: 4014516 kB
MemFree: 1466472 kB
Buffers: 76116 kB
Cached: 1026140 kB
SwapCached: 0 kB
Active: 1536520 kB
Inactive: 820268 kB
Active(anon): 1255464 kB
Inactive(anon): 127860 kB
Active(file): 281056 kB
Inactive(file): 692408 kB
Unevictable: 88 kB
Mlocked: 88 kB
SwapTotal: 4192252 kB
SwapFree: 4192252 kB
Dirty: 4276 kB
Writeback: 0 kB
AnonPages: 1254928 kB
Mapped: 164900 kB
Shmem: 128792 kB
Slab: 75096 kB
SReclaimable: 43516 kB
SUnreclaim: 31580 kB
KernelStack: 3752 kB
PageTables: 38476 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 6199508 kB
Committed_AS: 3750924 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 560444 kB
VmallocChunk: 34359174740 kB
HardwareCorrupted: 0 kB
AnonHugePages: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 63104 kB
DirectMap2M: 4096000 kB
Это работает на Debian, Ubuntu и RedHat.