You can make and use a contour gage if that's helpful to you. For my builds I use a straight edge. Generally either a 12" metal rule, or a 24" metal rule.
Lay the rule on the side of the forend and, using a light source from the rear, look at any "light gaps" that are appearing between the straight edge and your stock. Remove the high spots.
Once you have it even simply look at the stock from the front and that will tell you what shape your forend is in. If it's to fat....slim it down more.
Here's a pic of a half-stock I built when I was making and installing the nose cap. You can see that I already trimmed down the forend wood thickness to 1/8". After I installed the cap I then took my rasp\files and from the centerline of the stock, shaped the "roll" to the top of the wood by the barrel flat. Then do the same from that centerline to the bottom.
This will give you the round look and not the slab-side look. Again, use the straight edge during this shaping process as well. The straight edge won't lead you wrong.

