Badly damaged or missing shingles. Gaps, gaps, gaps caused by shrinking/swelling cycles around chimneys, vent pipes, etc. Anything that looks at all suspicious for letting water pass though gets silicone or blackjack from me. In spring and fall, we can have extended rainy periods where I live, so I error on the side of caution. It's a good chance to review the overall condition of the roof, too since replacing it is a major expense.

Check your roof at least once a year if you are physically able. Otherwise, get someone else to help with it. Your roof bakes in the sun all year long, so there is a lot of expansion and contraction. It is also subject to high winds, hail, and tree branches. Finding out there is an issue with it when a long period of rainy weather hits is a disaster. I had a kitchen ceiling ruined once because I was overly confident that my steeply-pitched roof would never leak, especially when the shingles looked fine from the ground.

Wow! I stand corrected. Great photo