Once can have monit monitor memory usage…

check system foo
  if memory usage > 95% then alert

Does it use free RAM, or free -/+ buffers/cache? (or something else?)

# free -m
             total       used       free     shared    buffers     cached
Mem:           998        851        146          0        114         70
-/+ buffers/cache:        666        332
Swap:         2047         54       1993