The dynamic loader finds shared libraries thanks to two files -- /etc/ld.so.conf and /etc/ld.so.cache. If you cat your /etc/ld.so.conf file, you'll probably see a listing that looks something like this:

$ cat /etc/ld.so.conf
/usr/X11R6/lib
/usr/lib/gcc-lib/i686-pc-linux-gnu/2.95.3
/usr/lib/mozilla
/usr/lib/qt-x11-2.3.1/lib
/usr/local/lib

The ld.so.conf file contains a listing of all directories (besides /lib and /usr/lib, which are automatically included) in which the dynamic loader will look for shared libraries.