If my tax return was garnished how do I find out who garnished it?

TurboTax does not receive feedback from the IRS or your state regarding money that was offset from your refund.  If you owed back taxes, child support, student loan debt, etc. you will receive a letter of explanation from the IRS in several weeks.  The WMR site will contain a message regarding your offset.  It is also possible that the IRS caught a mistake you made and reduced your refund.

It is also possible that your state reduced your refund for debts owed in your state—if the state reduced your state refund you will receive a letter from the state.

https://www.irs.gov/uac/tax-refund-offsets-pay-unpaid-debts

 NOTE: You can contact the IRS Treasury Offset Program Call Center at 1-800-304-3107 to ask if they have an offset for you on file. TurboTax would not have that information.

https://turbotax.intuit.com/tax-tips/tax-payments/who-can-garnish-an-income-tax-refund/L7cPPzDyc

 IRS Treasury Offset Program Call Center at 1-800-304-3107

To see back taxes owed 

https://www.irs.gov/uac/view-your-tax-account