New answers tagged stress
Anxiety and stress are very available states, and since you want to know how people currently feel (rather than some more implicit construct), you probably can't do much better than just asking them. A single-item anxiety measure as suggested by mlt should work.
Just found out that the upcoming Apple Research Kit for iPhone allows for some kinds of motor and touch activity monitoring. Maybe some of these can be re-purposed to collect data relevant to the original question.
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