(n.) One who swears to another's credibility.

TV-viewer abutter adjoiner attestant attestator attester beholder bird-watcher borderer cojuror compurgator deponent drugstore cowboy earwitness eyewitness gaper gazer gazer-on girl-watcher goggler immediate neighbor informant informer kibitzer looker looker-on neighbor neighborer observer ogler onlooker passerby perceiver percipient seer sidewalk superintendent spectator spectatress spectatrix swearer tangent televiewer television-viewer testifier video-gazer viewer voucher watcher witness