On Schedule E Worksheet, Line 18a (Depreciation) - where can I see how this value is being computed. I cannot locate the exact value on any other worksheet in TT.

TurboTax reports depreciation details for a business or property on Form 4562 Depreciation & Amortization Report (see screenshot below - click to enlarge). You can access this report using Forms mode in TurboTax for Desktop. See How do I switch to Forms Mode in the TurboTax for Windows software?

Depreciation calculations are based on the depreciable basis, life, and method. Using that information from the Depreciation report, you should be able to confirm the current depreciation in the last column. The specific details on how depreciation is calculated can be found in IRS Publication 946 How To Depreciate Property.

The amount from the Current Depreciation column is carried to Schedule Wks (Property Name) Line 18a. This amount then goes to Schedule E, Page 1, Line 18. If you have more than one property, be sure to look in the row/column for that property.