Q:

How was Abd al-Rahman I recognized when he arrived in Islamic Hispania?

Abd al-Rahman I was the founder of the Umayyad dynasty in Hispania, and he was one of the last Umayyad survivors when the Abbasid overthrew the Umayyad caliphate in Damascus. According to what I read, he fled Damascus, travelled west on low profile, and finally arrived in Hispania after 6 years missing. In Hispania he quickly amassed support, defeated the local Muslim ruler and installed himself as Emir. According to Wikipedia:

Abd-ar-Rahman I became Emir of Córdoba in 756; fleeing for six years
  after the Umayyads had lost the position of Caliph held in Damascus in
  750. Intent on regaining a position of power, he defeated the existing Islamic rulers of the area who defied Umayyad rule and united various
  local fiefdoms into an emirate.

I'm curious how the people in Hispania verified that he was in fact Abd al-Rahman, the Umayyad prince, because:

He travelled on low profile, so I guess this also mean without strong
troops or obvious indicator of being a royal
Hispania was pretty far from Damascus, so probably Hispanic peoples have less idea about how Umayyad princes look or how to verify them
He didn't seem to be an important prince, like the heir to the throne, he just happened to be one of the survivor.
Yet enough people in Hispania believed him to allow him to take over.

A:

Peter C. Scales:The Fall of the Caliphate of Cordoba: Berbers and Andalusis in Conflict (Medieval Iberian Peninsula, Vol 9), the source linked in the answer by
@Mr.lock / @Kobunite
actually hints at a plausible answer to OP's question, namely that 

Abd al-Rahman was recognized when he arrived in al-Andalus because members of the Umayyad family had already established themselves
there.

Relaying accounts by Muslim historians Scales says (p112) that already in 121AH/739AD
some 30,000 soldiers, among them 10,000 Umayyads, had been sent westwards by Calpih Hisham. 
A vanguard of this detachment, some 7,000 Umayyads, found themselves besieged by Berbers in Ceuta, and appealed to the governor of al-Andalus to be allowed to cross the Gibraltar. 
As he agreed these 7,000 would become the second wave of Arabs to settle in Spain.
So when Abd al-Rahman landed in al-Andalus 755AD he would be able to count on the support from his own kin.

