My wife does this exercise times 100. They have a storage bay full of Christmas decorations at the country club and around the community.
Her policy is to just chuck those little lights because there are several, incompatible, types of bulbs, it can easily take an hour to find the bad socket and they cost about $2.
If you do want to fix these, expect to find a socket that is not working because the little wire on the bulb is not touching the contact. They are usually shunted if they are just burned out. That tool they sell basically just cuts through the insulation and shorts out the bulb socket one at a time. It works if you only have one bad.
