When Lucas Desiree is beheaded in the episode "Innocent Man," his close friend Duncan MacLeod is nearby, senses the Quickening energy, and instinctively knows his friend has died rather than another immortal. This ability is not consistent with the rest of the series however, which repeatedly shows that immortals, even those with a close bond such as Duncan and Connor or Duncan and Amanda, cannot distinguish if they sense a friend or foe nearby or if a friend has died in a nearby battle and won't know until they make visual confirmation.