A sizzle is the sound worth checking: it can indicate the hot searing involved in wok hei. It is a cue to observe, not proof that wok hei is present.
How to check it
The cited cooking guidance advises starting a stir-fry with a clean, dry wok. During cooking, the reader can listen for a sizzle while checking whether the wok is overcrowded. Too many ingredients can cool the wok severely, and the guidance says that this shifts stir-frying toward stewing. The sound should be considered alongside the wok's starting condition and ingredient load, not as a stand-alone verdict.
What the reader must still confirm
The cited material gives no universal sound level or timing benchmark. The reader must still confirm the clean, dry starting condition, the amount of food in the wok, and the cooking heat. A change in loudness alone cannot establish wok hei.