For the first year it was used for business, look at Form 4562. Your vehicle should be listed in line 26 or 27. If there is depreciation in column (h), Section 179 in column (i) or if Line 25 shows the Special Depreciation Allowance for the vehicle, you used Actual Expenses in that first year.
If it doesn't show those amounts (but it shows your vehicles), you used the Standard Mileage Rate.
