You're looking for discolored, cracked or otherwise deteriorating caulk. Two important areas to check are your basement and the area around your windows. Both of these spots can leak a lot of heat, especially during the colder months of the year.

Angie's How To Learn How to Apply Caulk
Caulk may seem like something only professional contractors deal with, but it's actually inexpensive and extremely simple to use. By sealing areas around your house such as the attic, basement, windows and exterior siding, you can save as much as 30 percent on your utility bill. Best of all, a tube of caulk only costs about $4 at your local hardware store.
3 pros!
...with your putty knife or scraper.
... using the instructions on the gun package. Once it's properly loaded, cut off the tip of the caulk tube.
...pushing the caulking tub into the crack. Be smooth and use consistent, light pressure. You want your caulk to be applied in one continuous line.
Keep the spoon, drinking straw or popsicle stick wet to keep the caulk from tearing.
...before you cover it with anything (such as insulation).