A simple way to show the memory usage on a Raspberry Pi is to open the terminal program and enter
free -h -s 1
-s 1 requests an update every second. A table is displayed showing the total memory available, memory used and memory remaining. I take the memory used, divide by the total memory and multiply by 100 to get the percentage memory used. In my case running Icecast2 and MPD I use 72% of the available memory.