Cooling system problem

My car is daihatsu mira model 2000 900cc, the car was not used for 3 months, after that I used it for a weak without any problems, after that when I tried to overall check the system, I found the radiator coolant fluid is foamy and not clean so I went to a mechanic and flushed the radiator and filled the car with new coolant, after 3 days the car overheated up and motor engine sound changed as if it is going to stop but run again, it did the sound about 3 times in 10 seconds then the engine went off, I tried to ignite the car but the engine won't start, at this moment I heard the fan noise and noticed the high temp gauge, so I opened the hood to find that the car was completely empty of coolant water inspite of totally filling the car with the coolant 3 days ago, I waited for half an hour, then turned the car on and added the coolant, since that time this process occur every 3 days, and I can't find the leak, I over checked all connections but the problem is not solved, it was running the first week without any problems at all.
so what do u suggest, how can I find the leak if there is, how can I detect if the headgasket is blown?