How can you tell if an adult is choking?

They should be showing the universal sign for choking (two hands around the neck). Sometimes they'll be coughing instead, and if they are coughing, let them. That means that air is still going in and out and their body will try to get it out itself. It's not until you can't hear them breathing/gasping anymore. Sometimes their face will turn blue also, so just be prepared to act just in case.