ldd (Linux)

ldd prints the shared libraries required by each program or shared library specified on the command line. (use otool on Mac OS X)

Example: Want to know which Linux thread library java is linked to?

ldd /usr/java/jre1.5.0_11/bin/java
       libpthread.so.0 => /lib/tls/libpthread.so.0 (0x00bd4000)
       libdl.so.2 => /lib/libdl.so.2 (0x00b87000)
       libc.so.6 => /lib/tls/libc.so.6 (0x00a5a000)
       /lib/ld-linux.so.2 (0x00a3c000)