An easy way to find out about a possible leak is if you start getting an unusually high water bill or low pressure water when you open up the taps. Check the piping thoroughly and possible traces of fungi or green moldy stuff sticking around the pipe or soldering will indicate a leak. You can easily fix it at home yourself. In fact it is better to fix it as soon as possible before the leak grows and starts causing irreparable damage to your house.
Whatever copper pipe needs fixing, makes sure that it is devoid of any running water when you are going to fix it up. Turn off the water supply from wherever it enters the leaky pipe so that no water is able to flow into the pipe anymore. Open up all the taps and faucets that are linked to that pipe to get rid of all the water that was already present there inside the pipe and in the end place a bucket or some sort of container to collect all the dripping and leaking water.
Now before you start any mending work, you would want to clear up almost a 10 foot radius around the pipe of any dangerous possibly flammable items, any insulation, and all kinds of drop ceiling tiles, shower curtains or any sort of hanging or dangling objects like draperies.
Clean your pipe thoroughly now. Use a metal wool ball to scrape off and clean the copper surface of the pipe of all the dirt and grime and the mold particles. Run water over it to clean properly. Use a soft toothbrush or paintbrush afterwards to clean up the wool residues. After this use paint flux to coat the male part of the leaking pipe in order to get rid of any possible metal oxides that might have formed and collected there and would hinder the solder and the metal to flow together while you do the soldering task.
You will now work with a propane torch, so be careful and handle it with extreme care. Heat the insides of the female part of the joint or the pipe with the torch until it gets hot enough. Slide the other part through it slowly in one flow and keep it there until the metal melts and fuses the two pipes together. When it has cooled off, run the tap to check any drips. If there are none, your copper piping has been fixed.