It can be difficult to know when to replace your double-hung windows, even if they look okay. One simple test is to see if you have any drafts leaking from your windows or the surrounding frame. During colder months, it is easier to tell if you have drafty windows. If it is warm and hard to tell if your windows are leaking air, try taking a candle and holding it steadily in front of various parts of your window and window frame. If the flame flickers sporadically, you have a draft and it is probably time for new windows.
Additionally, you would do well to consider upgrading your windows if you have high heating and cooling costs. New modern windows will help you save money on utilities as they provide extra insulation.