You would either need to contact your former employer or use your prior year tax information to make this determination.
Basically, any amount of contributions to your Simple IRA that were considered post-tax or nondeductible would be part of your cost basis. So for any contributions that were made by you or your employer that were tax deductible on your returns in prior years would have no basis so the cost basis would be zero ($0). (This is the more common case.)
