What are signs that I must replace my roof?
In the Atlanta area the age of the roof is important. If the three tab shingle roof is fifteen years old you should be prepared for bad news unless you have a large canopy of trees shading it. Missing are loose shingles are an important visual guide to impeding trouble. Once a year carefully inspect the bottom of your homes eaves for rot. Note any portions that have a gray or salt and pepper look (impending rot). These are signs the roof is already leaking, just not into the house proper. If you have experienced significant hailstorms and you neighbors have been replacing their roofs, you too probably have hail damage (seven indentation per 10×10 sq. ft. by definition).
If visual inspection of the plywood or OSB board in the attic shows discoloration, you have leaks. Most roofing companies will inspect your roof for free in hopes of your business. Their recommendation will likely include a new roof though perhaps a repair might have sufficed.