What is the best instrument to use to check the signal quality of a CW or single-sideband phone transmitter?

An oscilloscope is the best instrument for visually checking the RF envelope of a CW or single-sideband (SSB) transmitter. By viewing the actual waveform, an operator can easily identify signs of clipping, flat-topping, or improper keying shapes that result in splatter and poor overall signal quality. Other tools are less effective for this specific task. A field-strength meter only indicates relative RF power levels, not distortion or waveform quality. A sidetone monitor only lets you hear the audio representation of your keying, missing what is happening to the actual RF signal. A signal tracer and audio amplifier only check the audio stages of the transmitter. They cannot detect issues in the RF amplification stages, where distortion like flat-topping most frequently occurs.