I've looked for sirens in Google Earth and Windows Live Local before. It's easier to see the sirens using the latter through the "Bird's Eye View" function.
You do have to know about general location of the siren you're hunting for so you can at least get the right streets on the map then look around that area.
Here's a pic of the location of my first siren; the Federal 500T on the corner of Summit Ave in East LA
