There are three main signs that indicate roof damage
Check for missing granules that have exposed the substrate, or black layer underneath them. The substrate exposed should look fresh, or not as if it has been exposed to the sun for an extended period of time. This will indicate new hail damage as opposed to anything preexisting.
Run your hand over an individual shingle and feel for dimples or indentations in it. These are an indication of hail hits. Press your thumb against the indentation and examine for a soft spongy feel, like a bruised tomato. This bruising may be hidden, but will cause the shingle to fail prematurely over time.
When big enough, hailstones can cause circular cracking around the hit itself in a half-moon or full circle shape. This sort of cracking indicates larger hail and a shingle that has been damaged through to the fiberglass matting, which can make it highly susceptible to water penetration.