I can give you the command to check it if you need.
Mainly:
ps aux
with "top" command your can check the cpu usage (wa, sys, user is usefull for me).
I have do that's with lot of mmorpg server:
http://pokecraft.first-world.info/wiki/Pokecraft_vs_other
I have downloaded you server, I will try compil it to check the usage at idle.