@Hammer5573, use water, as you would to raise the grain. That's about as effective a method for finding glue spots as I can think of. But, I've notice the lye seems to either penetrate of get under the glue, somehow. The last project I used it on was an experiment, which I didn't clean of glue as thoroughly as I should, and the color darkened even under the glue. Here is a pic, wher I was testing handle shapes (handle & lid aren't finished). The cherry edges are very dark, as I used a pretty strong lye solution. The walls of the box are white pine, and you can't see any if the multiple glue spots that exist.

@CoopI would look for a clean Sodium Hydroxide product. It is available from many internet sources, but for a quick experiment, you can look at drain cleaner labels, next time you go to the grocery. DrainO crystals, among others, is mostly sodium hydroxide. The recipe depends on how much reaction you want. 1/4 teaspoon to a cup of water will have a noticable effect, but may require multiple applications to reach the desired darkness. The more powder you add, the stronger the reaction, the deeper the color in one application, and the more caustic the solution. A teaspoon in 1/2 cup of water will make the water quite warm. DO NOT PRE-MIX!  The stuff doesn't store well after activated in water, and can corrode through many types of container.

@wtnhighlander, I think I will wait on the “ mad scientist” for a later project or better still, for the off cuts of these boards. What type of lye do you recommend and what is the formula? 

That’s sweet! 

I've been repairing a lot of walnut imperfections like in your pic, with epoxy and brown dye. Matches very well. Also try to save some walnut sanding dust from your random orbital sander, and mix that in with clear epoxy. That works, too. Don't forget to cut all the rotten, or soft wood, out. I use different shaped picks.