diff --git a/hgext/fsmonitor/__init__.py b/hgext/fsmonitor/__init__.py
--- a/hgext/fsmonitor/__init__.py
+++ b/hgext/fsmonitor/__init__.py
@@ -112,6 +112,7 @@
 import os
 import stat
 import sys
+import tempfile
 import weakref
 
 from mercurial.i18n import _
@@ -175,6 +176,23 @@
 # and will disable itself when encountering one of these:
 _blacklist = ['largefiles', 'eol']
 
+def debuginstall(ui, fm):
+    fm.write("fsmonitor-watchman",
+             _("fsmonitor checking for watchman binary... (%s)\n"),
+               ui.configpath("fsmonitor", "watchman_exe"))
+    root = tempfile.mkdtemp()
+    c = watchmanclient.client(ui, root)
+    err = None
+    try:
+        v = c.command("version")
+        fm.write("fsmonitor-watchman-version",
+                 _(" watchman binary version %s\n"), v["version"])
+    except watchmanclient.Unavailable as e:
+        err = str(e)
+    fm.condwrite(err, "fsmonitor-watchman-error",
+                 _(" watchman binary missing or broken: %s\n"), err)
+    return 1 if err else 0
+
 def _handleunavailable(ui, state, ex):
     """Exception handler for Watchman interaction exceptions"""
     if isinstance(ex, watchmanclient.Unavailable):
diff --git a/tests/test-install.t b/tests/test-install.t
--- a/tests/test-install.t
+++ b/tests/test-install.t
@@ -153,6 +153,16 @@
   1 problems detected, please check your install!
   [1]
 
+debuginstall extension support
+  $ hg debuginstall --config extensions.fsmonitor= --config fsmonitor.watchman_exe=false | grep atchman
+  fsmonitor checking for watchman binary... (false)
+   watchman binary missing or broken: warning: Watchman unavailable: watchman exited with code 1
+Verify the json works too:
+  $ hg debuginstall --config extensions.fsmonitor= --config fsmonitor.watchman_exe=false -Tjson | grep atchman
+    "fsmonitor-watchman": "false",
+    "fsmonitor-watchman-error": "warning: Watchman unavailable: watchman exited with code 1",
+
+
 #if test-repo
   $ . "$TESTDIR/helpers-testrepo.sh"