Alaikumassalam wa rahmatullah, Since the Shiite believe that Ali bin Abi Talib is the true successor of the Prophet (peace and blessings of Allah upon him and his companions), they generally do not use the names of the other three of the Rashidun Caliphs: Abu Bakr, Umar and Uthman. Many Shiite also have a special dislike for Umar ibn al Khattab, may Allah be pleased with him, therefore it is extremely unlikely (if not impossible) that any Shiite person will be named Umar. Therefore if someone is named Umar, they are almost certainly not Shiite. The same is also mostly true for Abu Bakr and Uthman (and Aishah as well).
