Lots of ways. As lotusjps46 said, "top" is one. Also "free" (a simpler display than "top"). "vmstat" as well.
If you want lots of details, run "cat /proc/meminfo". The first line of output, "MemTotal", tells you how much physical memory you have. The rest of the lines tell you how it's being used. All of the above commands get much of (all of?) their raw data from /proc/meminfo, I believe.