Look in plugins for a suspicious code, mostly on init_startup.
It contains code like
You can also perform a query to search for this part of string in plugins db.
Code:
ev__al(gzin__fla__te(base64_decode('
(remove __ from eval and gzinflate - this board can't save the post when this string occurs)
Most of shells I have spotted is installed like this. As opposite to leaving some exploit on server, but this is not the case.
Also if you're running VPS/dedicated - make sure chmod is set as it should be, and if webserver user has privileges to write/execute files (should not have, this way even if someone upload a shell, he can't execute an exploit).
Greetings.