help to determine market value?

Ive been reading on how to determine the market value but am still unsure of how to do this. Let me give an example of what im trying to figure out. I had someone call me about thier house they gave me all the information where it is price sq ft br/baths ect. From reading next i have to find out what houses are going for in that perticular neighborhood right, ARV to determine what to offer right? What is the best way to find the information im looking for? Is zillow a good site for this? I understand i want to make an offer asap when i find a good deal but need this information to know if it is a good deal so i need to be able to figure this information out fairly quickly. Can someone explain this to me please or point me to somewhere i can find how to do this with a detailed explanation. It seems like all the books i read dont explain this enough.