I think it depends how bad the leak is and where it is. If it's a slow leak and your losing your charge over a week or so, then it will likely be harder to find-- you may not leak enough oil out to see it. Look the system over carefully with a good light and see if you can see any oil spots.
Pros would use either a u/v dye that they'd put in the system, then find the leak with an ultraviolet light, or they'd use an electronic "sniffer" or "refrigerant leak detector" which can detect leaks by running it along the lines and fittings until it beeps when it detects the refrigerant. Actually the electronic detectors can be had for a lot less than you'd think-- $30-$50 can get one on eBay. That's probably less than the cost of a "diagnosis".