How do you report employer paid health insurance premiums on employee w2?

These are reported in block 12 with the code "DD". The W-2 forms being issued in January 2013 is required to include this data.

To calculate the dollar figure, include both the employer and the employees' contribution for health and dental insurance, as well as Flexible Spending Accounts.

This is informational reporting only, as of 2013. It's possible (and perhaps likely) that this information will be used to assess the "Cadillac" plan tax which starts in 2016.