Displays of mild disgust are typically less intense and of shorter duration than displays of extreme disgust, but even mild disgust is generally quite easy to spot, as it is very distinctive.
Signs include pulling back the upper lip (as one might instinctively do before vomiting), drawing the corners of the lips down and back into a grimace, and wrinkling the nose (as if smelling something bad). At the same time, the brow scrunches (the eyes squint and the eyebrows come inwards towards the nose — because some of the muscles that cause this are the same ones which scrunch up the nose).
The tongue may be moved forward, or actually stuck out between the teeth.
In more extreme cases of disgust (or hyperbolic, humorous expressions of disgust), people make vocalizations intended to echo sounds of vomiting (blech, ick, ugh, etc.).