How do I find out how much social security I've paid in

Each year the SS Administration sends out stateements of SS tax history and projections of expected SS retirement benefits based on your SS tax payment history.

You can also get this info on line; see: Retirement Estimator.

Note that the estimator does not as yet include data for 2011 SS taxes paid - for that information check your 2011 Form W2 which you should have recently received from your employer.