Knob and Tube in Old house

[Joe San]

Hello all, 

In closing right now for an old duplex, one of the issues that worries me is the house was built originally in the mid 1800's and although has been renovated many times I know it had Knob and Tube electric wiring in the house at one point.  The current owner is telling me that it has been updated to more modern service but I have to rely on his word.  I can see that its been replaced at least partially in the basement but I don't know how to tell about the rest of the house.  

Is it possible it was only replaced in the basement and the rest of the house still has it in the walls?  I am worried a regular home inspector wont be able to identify this and I could be looking for costly rewiring for the rest of the house in the future which would make it uneconomical for me to purchase and could kill the deal.  

I plan to also get an electrician to look through the home for me in addition to the regular home inspector, will the electrician need to break open walls to tell me for sure? 

Anyone with experience in this I would really appreciate it.  Thanks.