You use this tool:
https://socialsecurity.tools/app.html
But to use it, you must go to your Social Security account online (ssa.gov) and copy your SS earnings record. This is easy to do--just highlight the whole earnings record, and right-click "copy." Then paste that earnings record into the tool (it is useful to have two different browser windows open to do this; one for SS and the other with the SS tool page). This tool does not keep any information after you close out the browser window, so only you see the results. Once you have your earnings record in the tool, it will allow you to choose your final year of work, and it will then calculate what your
actual social security payment will be. The estimate will be very close to your actual payment, in current dollars.
One last note: Social Security is based on your highest average monthly earnings for 35 years (which is 420 months). If you have fewer than 35 years of earnings, any missing years will be averaged in as "0". Thus if you have 30 years of earnings, your social security average will have 5 years of 0. That will impact your ultimate benefit.