How do you evaluate Repair cost in a property

[Robert Blocker]

Im new to this, My question would be how would you actually determine your repair cost on a property before you make a offer. Do you physically go to the property or do you assess it based off what the seller tells you.? And should i pay someone to inspect it before i make a offer? But being in boston Not sure how evaluating rehab cost goes. Is there a number x sq foot?