Rule of thumb, if their head will fit, their body will follow. You will need to crawl around your entire house on your hands and knees, and look for small holes, crevices, loose ventilation covers, etc. You will need to check every room for these. Even if the ferret will be confined to certain rooms, sooner or later, they will slip past the barriers, and get into off limit areas, and it is better to be prepared for this, than to end up tearing your home apart looking for them. You will need to check over all appliances for holes and areas that the ferret could get inside of.
Ferrets can get up inside of some refrigerators and get hurt on the fan.
Ovens have insulation inside that can be ripped out, if it is a gas oven, there will be a pilot light. Dryers have tubing that looks like a ferret tube, ferrets can tear a hole, get inside, and even get outside through the exit vent. Cabinets may have holes inside that will lead inside of the walls, and ferrets can open cabinets. Couches can be very dangerous to ferrets. Most couches have a thin fabric lining underneath that ferrets will easily get through, and then up inside of the couch and the springs.