MacOS – How to find out how many bytes of memory the machine has in RAM

The System Information app has this information (and a lot of other info about your computer). Access it by going to the Apple menu (top left of your screen), selecting About This Mac, and clicking the More Info button.

Fire up the app and click on the Memory tab. You'll get a visual representation of how much memory you have and how it's installed. There is a rectangle for each memory slot, and each rectangle will be labeled "Empty", or with the capacity of the chip inserted.

For even more info, go to File-> System Report.
In the new window that appears, click Memory in the list on the left. It will show you the number of slots and what's in each one. In the below screenshot, there is one empty bank. It's the one with the Size, Type, Speed, and Status as Empty.
If none of yours say empty, you've got something in every slot.

Note that if you have an older version of OS X than Lion, you won't have the visual representation. You can open System Profiler (/Applications/Utilities) and follow the directions from the second half of this answer.

Check with this command (will show more processes):

sudo ps -awxm -o %mem,rss,comm | sort -nr | head

if won't help, check with fs_usage to display system calls and page faults related to filesystem:

sudo fs_usage -f filesys,diskio

especially page in and out by appending the following to above command:

| egrep -i "page_|pgin|pgout"

^{Add extra | grep -v kernel to ignore kernel process or | grep -v 0.00 to show these with higher time spent.}

If your swap is big, you can force disk cache to be flushed and emptied by: sudo purge.

Based on my experience, OS X have some issues with managing the memory after higher uptime (when it used all of free memory), so sometimes only full restart may help.

Check also sudo iotop or sudo vm_stat 1 commands which may help.

See also: How to investigate high kernel task memory usage?