A person with ADHD will find it very difficult to "sit still". You may suspect someone has this disorder because they always fidget to the point of distracting others. They may also have impulse control issues. They will struggle with tasks requiring focus although sometimes they will become hyperfocused on one thing. However, it is important not to confuse every lapse in attention or a temporary distraction with ADHD. These symptoms will have to be persistent and make it hard for a person to function with others.
Remember: ADHD is particularly hard to diagnose in adults.