There are some cheap MQ-3 sensors reactive to alcohol contained in air. I wouldn't expect them to be useful for anything else, but fun. The devices used by the Police need to be certified and regularly calibrated.
A few years ago, the Police (at least in our country) used chemical reaction that changed colour of a stripe included in an inflating balloon inflated by a person to be checked.
However, for precise alcohol measurement (contained in blood), a different method is used. Two samples of blood need to be taken after a while (not sure how the alcohol volume is measured) and then compared to some typical absorption curve to estimate alcohol level at the some specific time.
