The first thing to look for on a tar and gravel roof is any membrane that might be blistered up. This indicates that water has penetrated the membrane and has formed into steam inside the layers and has caused the membrane to buckle. Other signs of failure are splits or fractures in the felt membrane. This indicates that the roof has dried out and expanded and contracted too much putting stress on the system.
These problems can be addressed by making strategic repairs to the affected areas using hot tar and felt paper or bitumen membranes. Others signs look for are gravel ballast that has washed away exposing the underlying felts to Ultra Violet rays. Over time UV rays can break down felt membranes and cause the roof to fail. This can be fixed by adding a layer of asphalt and fresh gravel to the affected areas.
A good resource for more information can be found thru the CRCA