How can one check isnad?

To verify the hadith one have to verify isnad. My question is how to check Isnad? mean to say is there any early book of Islam (Authentic) in which the series of connection between narrator are written? How we know that Imam bukhari's father was taught by Imam malik and Imam Malik & Imam Abu Hanifa was taught by Imam Jaffar us Sadiq? or Hazrat Abu Baker (r.a) or Hazrat Usman (r.a) or Hazrat Abu Hurarah was companion of Prophet Muhammad (P.U.B.H)? Please don't get me wrong I am a Muslim but I am really confused that how the scholar of hadith even now a days can analyze the connection between Isnad? Please clarify with reference to the History books written during the golden era of Islam?