Look closely at the furnace while it's running. And those are all negotiation tools. Check the plumbing in the basement and under sinks for signs of leaks or bad joints. Press hard with your toe close to the toilet and tub to see if the floor is rotting. Ditto if there are a lot of armored cables around.
Wires spliced together without junction boxes signify sloppy electrical work.
Large cracks in the basement and garage floors could indicate foundation trouble. Look for off-plumb or sagging support beams and joists under the flooring overhead. Probe for rot in suspicious spots in the wood with your knife blade. Piles of dirty sawdust could mean carpenter ants. Make sure the attic is adequately ventilated, and that kitchen and bathroom fans are vented clear to the outdoors, not just into the attic air.