best way to detect invalid Haddock early?

I've had some CI failures because of invalid Haddock comments. What is the best way to detect such issues locally, early? I tried setting environment variable GHC_FLAGS to "-Winvalid-haddock", which should have told `./hadrian/ghci` to issue a warning. But for some reason, no warning was issued. What's a good way to detect invalid Haddock before it hits CI? Norman