Why would the Arch de Triumph (Titus’s arch) prove anything; wasn’t it made before he returned to Rome (with or without the menorah)? I always thought that the point was that the triumph (victory parade) would pass through the arch upon their return to Rome, and therefore it was made in advance (before they stole the menorah).
Still, people would know what the menorah looked like from when the Cohanim would bring it out to show it to the am by the Yomim Tovim (like in the end of Chagiga).