There are two ways that an A/C leak can be FOUND. The first diagnosis,
is to determine whether there is one. Checking the freon pressure (high
and low side) will give some indication (an evacuation test left over
night will be the best way).

Once a determination has been reached that an A/C leak exists a sniffer
can be used (depending on the Freon used.) I got one for the "propane"
derivative types at around $150, I think. Another common way is to use
a dye added into the freon. I believe the dye will glow with a UV
light, which the technician then can shine on the hoses to help
determine where the leak is.

Your symptoms sound like you need a recharge (and may have a leak) as
the gradual factor is definitely symptomatic of that.

Kim Brennan
86 Syncro. 91 Westy Syncro. 90 Tristar
Vienna VA. Sugar Grove WV.