Okay this time last year AC not cold anymore so take it to shop they say evaperator has leak and to replace it. So they do and have cold air for a while. Well this season air not cold so i put in can of freon have cold air last maybe a week do it again have cold air , so i have a leak how do i find it .

 

Yep you have a pretty good leak.
First you can do a thorough visual inspection. Check for oily/dirty residue at all of the connections, service valves, hose crimps, condenser face and on the compressor clutch and body. oil/dirt residue is a sure sign of a leak.
Your system had UV dye in it from the factory. You can rent a black light and yellow glasses from Auto Zone that will make it easier to see any leaks that aren't obstructed from view (back of condenser, evaporator, etc).

 

Thanks isrx01 i will look at it maybe it will be easy to find.

 

If the front seal of the compressor is leaking, you will most likely have to take off the clutch to see inside where the compressor shaft comes out. I would suggest doing so even if you find a leak somewhere else. If the front seal has leaked enough or badly, you may see the dye on the underside of the compressor body from front to back which means the front seal is leaking and the wind in the engine compartment is blowing the oil toward the back of the compressor. Might as well fix all the leaks and have a good sealed system rather then doing it again in a short time!