Ammonia has replaced fluorochlorohydrocarbons (Freon) as a refrigerant due to concerns about oxone layer depletion in the Earth's atmosphere. Only $2.95 for 100 strips.
Because ammonia (NH3) is a very small molecule, it can leak through very small gaps in the fittings used to attach gas lines. The image shows phenolphthalein test strips that changed color in response to the alkaline pH of ammonia. Note that the strip on the left which shows the response was wetted first with a drop of water while the one in the middle shows no change at all.
Simply wet the test strip with water & hold it near the suspected leak. The most likely places are typically joints or fittings. The paper will change color as shown in the image.
This is a fairly common test method for refrigeration units since strips are cheap & portable. They can be used at home, in hockey rinks & everything in between. For more precise quantitative detection, there are special gas detection units for this purpose.