The Goosegg, is a machine once used by NASA in the 1970's to treat the Apollo missions astronaunts eye sight when they suffered from white spots, it was later perfected and used in hospitals to monitor brain activity in comatose patients. Henry Darrow in To Ride Pegasus, was the first Talent to have it used on him when Molly Mahony took an intrest in his brain waves, as they spiked right when he was having a vision of the future, which was revealed to be him seeing himself in the future married to Molly. Aftwards, Molly scanned her own brainwaves and compared hers when healing to that of Henry's to Dr. Wahlman, which he could not explain vey well, to prove that Talents set off a distinct higher brainwave patterm, when using their powers; it is later used to tell who is really a Talant vs who is a fraud, to be able to live at The Center. When a patient is hooked up to a Goosegg, a chart is printed, which is called an EEG or its official name Electrocephalograph.
Electoencephalography article on wikipedia