This isn't exactly a method of measuring chainline, but it is very simple a way of making sure it's straight... I just look down at the chain from above and make sure it's parallel with the top tube of my bike. If you look down from a bit in front of your handlebars, you can visually line up the whole length of chain with the top tube. I think it's possible to detect if the chain is off by 1mm if you're careful.
It doesn't work as well for bikes that have very sloped top tubes, though.
Here's a view of my bike from the top -- this photo doesn't show the chain and the top tube lined up as closely as they can be, but it should give you some idea of what I mean.