When using a digital mode based on a computer sound card, how can you verify that the transmit audio level is NOT excessive?

When operating sound card-based digital modes, it is crucial to avoid overdriving your transmitter's audio input. Excessive audio levels will cause the transmitter to operate non-linearly, resulting in intermodulation distortion, commonly known as 'splatter.' The best practical way to ensure your audio level is not excessive is to ask a local station to observe your signal on a waterfall display and confirm it is clean and free of splatter. The other options represent exactly what you should avoid. You do not want audio compression turned on, nor do you want the Automatic Level Control (ALC) actively limiting the transmission, as both will heavily distort a digital signal. Furthermore, simply confirming a signal can be decoded does not mean it isn't simultaneously splattering across the band.