You can calculate the amount of your refund throughout the year, not just at tax time, by using a refund calculator. The Withholding Calculator can be used for this purpose. You simply input your withholding allowances, estimated income, and the calculator automatically calculates the refund associated with that particular withholding amount.
You can get an estimate of your refund by adjusting the amount of your withholding. Using this refund calculator can actually be informative. It will show you how your refund can change according to your withholding amounts. If you are self-employed (and have no withholding taxes) it also will show you your refund. You just need to have an estimated income amount and deduct the estimated taxes you have paid throughout the year.