As oxman said, you only need to look at nearby lines that are similarly tabbed. There’s a Java applet at this page that lets you play around with elastic tabstops.
I’ve actually been thinking about making a Sublime plugin that replicates the behavior of elastic tabstops through capturing the tab key and replacing it with a certain number of spaces, then looking at nearby lines and modifying them. If I find some time I’ll throw something together. This use of spaces instead of tabs would also mean that the text would be completely portable into other editors.