In order to plan for retirement, we tracked our spending, to the penny, for about three years. We created spreadsheets and manually went through checkbook and cc's to categorize the spending. Then, we did our best to allocate anticipated expenses and include them on the spreadsheets. For instance, tried to determine an amount for future car repairs, or loan on a new car, or house expenses such as a new roof (which so many people fail to anticipate, according to so many studies). We were then very confortable determining how well we could retire and spend money on traveling and other activities.
There is no way to answer the oft-asked question: "Can I retire", without going through the exercise of tracking current expenses and making educated guesstimates of future outlays. Of course, future income is determined as best as possible too.