For the most part, when freon leaks it will leave a trail of oil (PAG) where it escapes with the pressure. If the leak is very small, in time the oil will saturate that area and accumulate dust and will give you a clue where the leak could be coming from. I would suggest that the next time you have the a/c system recharged, have them install an a/c dye into the system. A reputable a/c shop would have a UV light that can detect the dye when it leaks out the a/c system and determine where the leak is. For a preliminary check of your own, you can inspect the condenser,accumulator, a/c lines and their connecting joints(fittings) and a/c compressor and see if anything is saturated wet from an oily substance. Keep in mind of it's surrounding components that may also lead you to a false assumption of a leak such as a transmission line leak that can saturate it's surrounding area with the fluid or and engine oil. If no leaks are found in any of the areas readily accesible , chances are, the leak could be in the evaporator which could be diagnosed by someone who has a UV camera system to verify inside the hvac housing.