First of all, I ought to appreciate the response of @Muhammad Abdullah as a relatively nice answer. In truth, he termed some characteristics and signs regarding Imam Mahdi and reappearance which could be helpful for you in order to identify the real Mahdi (A.J.). There are some or even several characteristics which together give us certainty concerning actual Mahdi (A.J.). Briefly speaking, I name some of them.
It is narrated that Hazrat Qaem (Imam Mahdi (A.J.)) reappears, (next to the Kaaba) he will stand between Rokn and Maqaam, and says five sentences:
الأول: ألا یا أهل العالم أنا الإمام القائم
الثانی: ألا یا أهل العالم أنا الصمصام المنتقم.
الثالث: ألا یا أهل العالم إن جدّی الحسین قتلوه عطشان.
الرابع: ألا یا أهل العالم إنّ جدّی الحسین علیه السّلام طرحوه عریانا؛
الخامس: ألا یا أهل العالم إنّ جدّی الحسین علیه السّلام سحقوه عدوانا
Source:
یزدى حایرى، على، إلزام الناصب فی إثبات الحجة الغائب (عج)، محقق، مصحح،
عاشور، على، ج 2، ص 233، بیروت، مؤسسة الأعلمى، چاپ اول، 1422ق.
On the other hand, it is narrated that:
it will be heard a heavenly (celestial) call which it has been
interpreted as Saihah or Sowt as well. (Apparently it would be at the
night of Friday and likewise it will be coincided with twenty third
night of Ramadan. And everybody on the earth will hear it.
Source: شیخ صدوق، کمال الدین و تمام النعمة، محقق و مصحح: غفاری، علی
اکبر، ج 2، ص 650، دار الکتب الاسلامیة، تهران، چاپ دوم، 1395ق. ر. ک:
علائم ظهور، پاسخ ۴۹، و بخش شیعه چه میگوید در سایت اسلام کوئست ذیل
«امام مهدی».
Furthermore it is quoted that there are five certain signs for His coming that include:
- The rise of Seyyed al-Yamani
- The rise of Sofiani
- The heavenly (celestial) call
- Murder of the pure soul (nafs az-zakiyya)
- Subsidence of the land of Baida’
Source: کمال الدین و تمام النعمة، ج 2، ص 650.
Citing attributes regarding his apparent form would be helpful as well. Al-Mahdi looks:
- Good looking
- Middle length (not tall, not short)
- Sweet-smelling
- High-direness (awe-inspiring)
- Having a young face (despite being aged)
- And more...
Source:
اکمال الدین، ج 2، ص 65۲ / [8] نک: موعود شناسی ص 437
Conclusion:
By paying heed to the above-mentioned items and apparent characteristics, you’ll be more certain about that. Good luck mate.
References:
www.islamquest.net (1)
www.islamquest.net (2)
www.islamquest.net (3)