I use one of two ways, depending whether the bait is one part or jointed.
For one part I use papertrick, shown below.
First, I trace the bait to a peace of paper and cut it out.
Then I fold it in half, find the balancing point and press my pen through.
Then I mark the point to my bait. Easy way to find center of gravity for one part bait.
For jointed baits, I always find the CoG for each segments. I could use this paper trick to find the center but then there's an weight distribution problems...
So I use this method below instead.
I'll take an rubber band and twist a small wire on to it. You can also use fishing line. I'll loop that rubber band on the segment of my bait and start adding weights to that wire. I can change the distribution of the weights by moving the rubber band to front or back, or adding a second one, like shown in pics below.
Weight in front of the CoG.
Weight behind the CoG.
Almost perfect weight distribution.
Then I mark these spots, drive holes and add these weights.