I don't really like spam assassin in all honesty but I thought it ran all the time not only when new mail came in. I know that it spawns a child process when new mail comes in to check it but there should be a daemon or parent process running that sees the new mail and spawns it.
With some control panels the process name may be different than spamd, it's possible that your hosting company can answer that question but you may need to use ps to figure it out.
first get a pid of a running spamd process
Code:
ps aux | grep spamd
Then get the parent process ID of it:
Code:
ps -F -pinsert process id here| cut -d" " -f7
Then
Code:
ps -f -pinsert parent process id here