How to assert if a std::mutex is locked?

With GCC 4.8.2 (on Linux/Debian/Sid 64 bits) -or GCC 4.9 when available - in C++11- I have some mutex

std::mutex gmtx;

^{actually, it is a static member in some class Foo containing both alpha and beta methods below.}

it is locked in alpha like

void alpha(void) {
   std::lock_guard<std::mutex> g(gmtx);
   beta(void);
   // some other work
}

and I want to check in beta that indeed gmtx is locked:

void beta(void) {
   assert (gmtx.is_locked());
   // some real work
}

^{(notice that is_locked is only called inside assert... It can be very inefficient or even sometimes inaccurate)}

Of course, I have other functions calling beta, e.g.

void gamma(void) {
   std::lock_guard<std::mutex> g(gmtx);
   beta();
   // some other work
}

but is_locked does not exist.... How should I define it? (actually I would like to be sure that the mutex has been locked in the same thread by some [indirect] caller...)

(the reason I want to test that with assert is that beta could be called elsewhere)

I cannot use try_lock (unless using recursive mutexes), because in the common case it would lock an already locked mutex... (locked in the same thread by a caller) and this is not only undefined behavior but blocks entirely.

I want to avoid recursive mutexes (more costly than plain mutexes) unless I really have to.

^{NB: The real program is a bit more complex. Actually, all the methods are inside a class which maintain a naming bi-directional relation on "items". So I have inside that class a map from items to names and another from names to items. beta would be the internal method adding really a naming, and alpha and gamma would be the methods finding -or adding- an item by its name, or a name by its item.}

^{PS: the real program is not yet released, but should become part of MELT - its future monitor; you can download it (alpha stage, very buggy) from here (a temporary location)}