My knowledge about STDs and STIs is minimal. All my knowledge is through internet where i read that they can spread through saliva as well. Not sure if that is right or not. Should i go and get tested for that? if yes, then what tests are we talking about? 

Welcome to the Forum.  I'll be glad to answer your question.  There are no STIs, including HIV which can be transmitted from person to person through sharing a cigarette or from eating or drinking from the same utensil.  There are recent data to suggest that gonorrhea may be transmitted from person to person through open mouth, deep kissing in which saliva is shared between the two participants but how common that occurs is controversial and most experts, including those of us on this forum feel that such transmission is a relatively rare event.  

You are not at risk for any STI from sharing an e-cigarette.  There is no need for testing or for concern.  I hope that this comment is helpful.  EWH