Does your unit have a sight glass? If so, look in it with the unit running and check for bubbles. Note that while bubbles indicate lost charge, you often have to check the glass after running for a few minutes to stabilize operation.
If there isn't one, ask a technician to add one in and fix the leak (if there is one) at the same time. It's only about $10 and very useful for troubleshooting.
BTW, although a leak is the most likely problem, keep in mind that a plugged up filter or bad expansion valve can cause the same. There should be 2 pipes to the condenser. Find the one that goes to the filter. The filter looks like a barrel and goes before the sight glass or expansion valve. (There might be a liquid line receiver although the filter often doubles as a liquid line receiver.) If the filter feels much cooler on the outlet side than the inlet side when running, it's bad.
