You need to have a REPUTABLE AC shop do what's called a "sniffer" test. Actually to be thorough they should refill with UV dye then check at a later date with both sniffer and UV lamp to be certain.

I have a slow leak in my W124 and lack the $$$$ to fix right now. I top of 2-3 times a year. I've looked into trying leak fix it type products but everyone here says to stay away from such quick fixes as they tend to ruin your other AC components. Don't know what the truth is but I think I'll be saving my pennies for the big evaporator fix.