A boiler's temperature and pressure valve might leak if it's not functioning correctly, and it might show visual signs of damage or corrosion, especially if it's an older valve. However, one of the biggest dangers of safety valves is that dangerous obstructions might not show any signs.
Because small water leaks are common on safety valves, corrosion presents a risk, especially to older units that have not been maintained properly. Small amounts of rust can form and spread over time, potentially threatening the integrity of the valve. Minerals in the water can also cause corrosion, although a greater threat is posed by mineral deposits that can prevent the valve from opening if the system is overheated or holding too much pressure.
Safety valves typically have a "test" setting, and many operation manuals recommend testing the valve at least once a year. Testing the valve, however, can present risks, especially if the valve has not been tested in a while. If the valve is corroded, testing it might cause a leak that cannot be repaired easily, which can flood the room with hot water. Testing the valve can also cause water or steam to spray, which presents a scalding risk. While expert inspections can be expensive, experts know how to test the system properly and what to do if the test fails.