As stated in earlier posts how to interpert the signs and the correct use has a lot to do with how good any detector is. We have all types electronic, ultrasonic, UV dye, halide lamp, soapy water (Snoop, Bubble Up etc) and sulphur sticks. It depends on where we suspect the leak is, inside a building, inside a condenser / evaporator or outside, they all have their uses.
If you use soapy water a tip is to add some glycerine to the mix it helps it cling to the pipe longer.
Ultrasonics work on all refrigerants even NH3 but will pick up other things other then a leak, Electronics pick up other things as stated before, UV dyes are messy. All have advantages and disadvantages. I never rely on one alone.