How to find the source of a water leak?

I live in a condo. Last year on July 4th my ceiling began to leak with water. The plumbers came out and could not find the source. Then a company with a little camera came out and said the source was the upstairs neighbors' water heater overflow was not connected properly. So it was fixed. The leak happened again this year on July 4th! Again the plumbers can't find the source. The leak is not constant. It will drip for 3 minutes and then not drip for days. How do I find this leak???