There's no safe way I'm aware of to test if the wires installed are able of supporting the safety equipment that's connected to them, short of physically verifying the gauge of the wire. In fact, many fixture wires can't support 15-20 amps even though they are plugged into a receptacle or attached to an outlet that can. These wires rely on the fixture or appliance not pulling more than the wire safely provide. E.g. you don't wire the inside of a 5 watt night light with enough copper to support 20 amps because the night light itself shouldn't pull more than 0.05 amps. From the perspective of the circuit breaker, there's no way to know the difference between the night light and the wiring inside the wall.
If you are worried about improper taps between circuits, you can try shutting off all the breakers, and then one by one turning on a breaker and checking for voltage on the hot wire of all the other powered off breakers. You can also shutoff power to all but the high amp breakers and check for any lights or receptacles that are receiving power. Both of these test should be performed with a non-contact tester if possible.
The clamp on amp meters are good for determining why a breaker is tripping, or to verify that it is tripping when current exceeds the safe minimum, but less useful to tell if there's an illegal tap since current needs to travel through the wire to be tested. It could be placed on an outlet with a high usage to see if that usage exceeds the safe minimum. But if you measure at the breaker and a separate circuit has been tapped, it wouldn't tell you that power is traveling through that other circuit.