diff --git a/docs/devel/testing.rst b/docs/devel/testing.rst
index 8a9cda33a5..8359f2ae37 100644
--- a/docs/devel/testing.rst
+++ b/docs/devel/testing.rst
@@ -224,6 +224,14 @@ another application on the host may have locked the file, possibly leading to a
 test failure.  If using such devices are explicitly desired, consider adding
 ``locking=off`` option to disable image locking.
 
+Debugging a test case
+-----------------------
+The following options to the ``check`` script can be useful when debugging
+a failing test:
+
+* ``-d`` (debug) just increases the logging verbosity, showing
+  for example the QMP commands and answers.
+
 Test case groups
 ----------------