Device to detect where heat or cold air is escaping or entering a structure
A thermal imaging gun will show you all the hot and cold spots inside your house, but they are very expensive.

What I did was I purchased a infrared thermometer gun at my local harbor freight. It cost about $15.00 Then I went around took all the temperatures throughout the house.

I found in my basement the rim joists were not insulated, I also found my attic hatch was not insulated. After doing that and caulking the windows, the cost for the insulation and caulk paid for itself the first winter.