My flock() manpage (linux) says (emphasis mine):
Locks created by flock() are associated with a file, or, more precisely, an open file table entry. This means that duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer to the same lock, and this lock may be modified or released using any of these descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate descriptors, or when all such descriptors have been closed.

A process may only hold one type of lock (shared or exclusive) on a file. Subsequent flock() calls on an already locked file will convert an existing lock to the new lock mode.

So you'll probably have to run some kind of external process to test the lock. Or you could decide to trust that flock() works.

Also of interest - from the perlfunc flock() manpage:

On systems that support a real flock(), locks are inherited across fork() calls, whereas those that must resort to the more capricious fcntl() function lose the locks, making it harder to write servers.
"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re: Testing if your code successfully flocked by Joost
in thread Testing if your code successfully flocked by dragonchild

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