knob and tube is still acceptable for existing installations. Things to look out for are if there has been any additions to knob and tube circuits which likely were done incorrectly. Also, ensure the K & t branch circuits are GFI protected. Another potential problem is lighting fixtures, where the insulation will likely have broken down due to heat. All you can do is inspect all outlets, examine whatever you can in the attics etc, identify the fact that the house would not meet current code and clearly state the inspection is that only - ie it is not a performance guarantee or certification of safety.
If you read the fine print on some of these home inspector reports, they absolve themselves of any resposibility if they miss something. Even a fire alarm system inspection states the devices were working at time of inspection, if it is not working the next day the inspection company can say someone may have tampered with it after the inspection.Make a list of every outlet in every room and main service, and state what you have done with it during inspection. For example remove, visually inspect, confirm voltage, confirm overcurrent protection, number of outlets on circuit, condition of wiring and device. Final comments would include where the residence does not meet current code, any glaring safety issues you see and recommendations.