Editors know that a python line ending in a colon must be indented and so indent it automatically. It wouldn't be hard to use the same method to reindent it.
But the OP's point is that you can't know how many lines make up the indented block, since there is no explicit block-end-marker left if the indent is gone.
Except if you have, for example, an if statement. if x: y() is valid.