Updated patch with the heuristics factored out into a helper function, with a more detailed explanation and additional logic to handle compound statements.

>>> def foo():
...     print bar
  File "<stdin>", line 2
    print bar
            ^
SyntaxError: Missing parentheses in call to 'print'

It's still just basic string hackery, though. The one liner handling, for example, relies on the fact that ":<whitespace>print " and ":<whitespace>exec " are going to be uncommon outside Python 2 code being ported to Python 3, so it just looks for the first colon on the line and checks from there, without worrying about slice notation or dicts.