If we didn't have an assessment, can we use a linear method to determine the adjusted cost base for the capital gains on a condo that we lived in then started renting?

There is no Straight line depreciation  allowed on Canadian tax returns for Real properties. For depreciation ACB  for Canadian property you need to use FMV of the property at the time  of  deemed disposition from personal use to rental. 

If you converted property in 2016 or 2017 you also need to report deemed disposition  of your principal residence on your tax return.