Try turning on warnings in ghci. This enables the compile time warnings you can get with ghc by passing -W, for example. You can do this several ways:
ghci -fwarn-incomplete-patterns
Or Neil Mitchell describes how he sets this up in his .ghci. Here is the relevant excerpt:
:set -fwarn-incomplete-patterns
You can manually enter this at ghci as well, but it would be a pain to do so each time you start it. Entered this way, it only works for statements entered at the prompt, not for loading files with :l. Instead you can put this comment at the top of the file you want to warn about incomplete patterns:
{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}