How to pray if not sure imam is traveling or resident
Question

                If a traveling person comes upon a congregation praying, and he does 
not know if: (1) the imam (prayer leader) is a traveler, meaning that he 
should join the prayer with the intention of qasr (shortening prayer), or 
(2) if he is resident, and thus should pray the complete prayer, what 
should he do?
Praise be to Allah.The answer that is apparent is: he acts based on what is clear 
  to him from the signs of travel on the imam from clothes or travel gear. 
  If it appears to him that the imam is resident, then he should pray the 
  complete prayer.
The evidence: what Imam Ahmad authenticated and narrated 
  that Ibn Abbas was asked: "What is reason that the traveler prays 
  two rakaat (iterations of prayer) if he is alone and four rakaat 
  if he is led in [a full] prayer by a resident? So he said, that is the sunna 
  (tradition of the Prophet ), 
  and in another narration "That is the sunna of Abi Al-Qasim."
Take the case of one who assumes that the imam is a traveler, 
  and thus prays with him two rakaat with intention of qasr. Then, 
  after salaam (completion of prayer) with the imam, it becomes 
  clear to him [from speaking with others after the prayer, for example] that 
  the imam is actually a resident and that the two rakaat that he 
  had prayed were the imam’s third and fourth. In such an instance, 
  he should stand up and make two rakaat to complete the prayer, he makes 
  sujood sahw (an extra two prostrations). Whatever speaking was 
  done or questions asked to investigate do not harm his prayer.1
From the book What Should You Do in 
  the Following Situations... ?
1 Al-Majmooa lil-Nawawi 4/356