One clear sign is when your cost per conversion keeps going up even though your budget has not changed. Another is when your ads are showing for keywords that have nothing to do with your business, which means your targeting is too loose. If your click-through rate is very low, it usually means your ads are not matching what people are searching for. A campaign that has too many keywords crammed into one ad group is also a sign that things need to be reorganized properly. When you see these things happening together, a full rebuild is often the faster and smarter move. What signs pushed you to restructure a campaign before?