I'm not 100% knowledgeable about pthreads-on-linux, but the Linux kernel does NOT distinguish between threads and processes. They are all treated the same internally, except when created. So, one can create lightweight threads (eg. kftpd, khttpd), but they will appear to be ordinary processes when queried.

You might be able to distinguish between them by looking that the process flags, possibly available somewhere under /proc/$pid

-ml