If you know for an absolute fact that you have not depreciated the property over the last 10 years, your issues can't be cleared up by simply doing now. You need to seek professional help, as a failure to depreciate rental property will make reporting the sale more complicated.
If you have been using TurboTax all this time then most likely you have been depreciating the property and it's perfectly possible you don't even realize it. If you have been entering data correctly over the years, the program has done it for you. If you report the sale in the SCH E section of the program then the program will take care of the recapture "for you".
To get the total depreciation taken you can use the IRS Form 4562 titled "Depreciation and Amortization Report" from the last year you actually rented the property out. Add together the amounts in the "Prior Year Deprec" column and the "Current Year Deprec" column and the sum will be the total of all depreciation taken on the property.
This gets a bit more tricky if in any single year you were required to take AMT (Alternative Minimum Tax) depreciation. Reporting the sale is also a bit more involved if your depreciation value was less than your actual cost basis. In such a case it would matter if you sold the property at a gain or a loss.
