Noveflyer, take a couple of pieces of 2X4 or similar and cut them to the same length, 12-14 inches is good. Nail them to a base, I used 12X12" pieces of shelving stock from Home Depot. Drill a hole in the center of the other end of the 2X4's about the depth of a drill bit and insert a couple of pencils, one in each 2X4 making sure they are the same height and you have a crude but very effective CG checking device. I use a similar method to check everything I have.

Just set the wingtips on the eraser end of the pencil and adjust for level. Hope it helps.