There are definite signs before the reappearance of Imam Al-Mahdi (AS) mentioned in the authentic narrations e.g. in Kamal Al-Deen by Shaikh Al-Sadouq (2: 650) from Imam Jafar Al-Sadiq (AS): Defibite signs for the reappearance of Imam Al-Mahdi(AS) are: Al-Yamaani, Al-Sufyani, Al-Sayhah ( The loud Cry from the sky, killing of Nafis Al-Zakiyyah ( the pious person), and the land slide in the desert.
'There are other narrations including the Dajjal.
'Wassalam.