Some home sellers will attempt to hide signs of mold and mildew by applying fresh caulking on top of existing caulk strips. Be on the close lookout for this. For starters, old caulking should be completely stripped before applying a fresh layer. A poorly-applied layer indicates that the owner was trying to hide signs of mildew in the quickest way possible.
Other things to observe while home shopping are air vents and windows. Is there enough ventilation inside of rooms such as the the kitchen and the bathrooms? Look around the air vents for signs of mildew spots. Of course, check the plumbing areas such as the sinks and the faucets.