airbuswiz,
Easiest, simplest, cheapest, most accurate I've found is a string/rope and plumb-bob.
Cut a string (with a loop on each end, .40 sized would be about 5 feet long) with a loop at each end large enough to go over the rudder/rear of fuse and a loop large enough to go around the prop hub. Make yours larger/tie around fuse instead of under rudder. Now tie another string around this string, so that it will slide, and you can move the center point so that plane will hang level.

Here's where I run this through a hook on the ceiling, and lift the plane off the ground. Tie off string to appropriate non-moving object.
You now have a plane with a string that is tied to its' nose and tail, with another string tied somewhere near the center of this string (really, it's right above the CG), that is holding the plane off the ground (not too far!)

Now tie a plumb bob on a string to the string that passes through the ceiling hook, and adust the plumb bob so that it hovers just over the plane. The Plumb bob now points right THROUGH the CG. IOW, the CG lies directly below the plumb bob.
Hope this helps.....but you've probably seen this thing used before. I'll send a picture if you haven't seen it, there's NO WAY you could build it from my crappy description!

Joe Myers