In decide_allocation function some element may when to test the proposed allocator.
For example like this:
if (gst_query_get_n_allocation_params (query) > 0) {
	GstAllocator * allocator;
	GstAllocationParams params;
	gst_query_parse_nth_allocation_param (query, 0, &allocator, &params);
	if (g_strcmp0(allocator->mem_type, GST_ALLOCATOR_DMABUF) == 0)
		GST_DEBUG("got dmabuf allocator");
	else
		GST_DEBUG("got an other allocator");
}

https://bugzilla.gnome.org/show_bug.cgi?id=703659