OK, based on what I've experienced before:

1. Check if you reservoir is full when your radiator is empty. If there's a clog somewhere or your radiator is overheating, water will tend to pool in the reservoir tank. Could mean the water pump, the thermostat or clogged tubes.

2. Check if the radiator tubes are rubbing against any metal parts. The create minor tears in the tubes which will not be visible when the coolant is cool. Water will only start leaking when everything gets hot. Drive around until the radiator fan kicks in (which means the system is already hot) then look for water leaking anywhere. That's how I spotted mine

Hope the tips help