I have always found it best to cut a piece of fish tape to the size of a speaker template and poke a very small hole in the ceiling and poke around before cutting any holes.

I know it isn’t that fancey but it works great when someone else has done the pre-wire.

We use the flexible rod that comes with the Wet Noodle from LSD, http://www.lsdinc.com/content/product_details/62

Bend it to the right length, the rest is the handle.  Push the radius length section into your exploritory opening and rotate.  If something is in the way, it wont rotate 360°.  Saves drywall patching.  Do this before you scribe your circle too so you don’t have to try and erase your pencil marks.