How can I tell if my overpaid social security tax has been reflected in my federal tax return?

If you had two or more employers in 2018 and the total Social Security taxes withheld were greater than $7,960.80 then the excess taxes are entered on Form 1040 Schedule 5 Line 72.  The total from Schedule 5 flows to Form 1040 Line 17 as a tax payment.  The excess taxes withheld are included in the tax refund or reduce the taxes owed.  It is not a separate tax refund.

Using the online editions you can view your Form 1040 at any time.  Click on Tax Tools on the left side of the screen.  Click on Tools.  Click on View Tax Summary.  Click on Preview my 1040 on the left side of the screen.