Is the kid going to throw up? "Read" his lips.
Hopefully the usefulness of Michelle's hack will make up for my horrendous wordplay in the title:
Like a lot of your contributors, this didn't really dawn on me until recently to be something that might not have occurred to others:
When my kids are sick, a tell-tale sign that they're going to barf is when their lips lose all colour. If they're complaining of an upset stomach or some other malady but their lips are bright red, I can usually ease back & look for the ginger-ale or other stomach-calming solutions. If their lips are pale and thin, I run (don't walk) for the bucket. If I ask them directly, they are usually in denial about throwing up (much like myself when I'm sick! "if I just lie still for a while...")
Ha! With my kids, the tell-tale sign is a pale face and sudden dark circles under the eyes.