This item is the foundation of what makes a successful retirement happen. You must have a really good understanding of what your revenue flow is. When you are w*rking this is easy as you can always look at your weekly or monthly paycheck and go from there.
When you retire that paycheck no longer comes in and the income it brought must be replaced? How will this happen? Well hopefully by the time you retire you will have alternative income streams available. These could be in the form of pensions (not likely anymore), IRA savings, 403B's, 401K's, rental income, stocks, savings or many other income producing instruments.
You must sit down and collect all your income streams and put them on paper or in a spreadsheet for further analysis. Will these income streams provide you with the necessary income for retirement for the "X" number of years that you will be alive post-retirement. If not plan now for the future and start saving or investing wisely to provide for your retirement.