From the front and rear
Look for changes in the roof plane, moved coverings, broken ridge or verge material, blocked valleys and gutters that are holding debris.
At chimney level
Scan pointing, pots and visible leadwork. From the ground you may not see the back of the stack, so note what is hidden rather than assuming it is sound.
Inside the loft
After rain, look for fresh staining, wet insulation and moisture on rafters. On cold dry mornings, widespread droplets may suggest condensation.
Keep occasional photographs
Pictures taken from the same safe viewpoints make slow changes easier to recognise. They are especially useful after storms.