How do I know if I got credit for my Service in the Army (1967-1969 Vietnam) on my Social Security Benefit?
There's really no simple way to verify that you received credit for your military service. What you should have received credit for are deemed military wages (DMW) in addition to your actual military wages for the periods of time that you served on active duty (https://www.ssa.gov/OP_Home/handbook/handbook.09/handbook-0953.html). The combined amount is then used in the normal Social Security benefit calculation formula (https://www.ssa.gov/pubs/EN-05-10070.pdf). Starting with 1968, DMWs are automatically added to a person's Social Security earnings record, so it's a virtual certainty that you would have received all of your credits from 1968-on.
When you file an application for Social Security retirement benefits, one of the questions on the application asks if you had any active U.S. military service from 1939 to 1967. Assuming you answered that question affirmatively, Social Security should have asked you for proof of your active duty (e.g. form DD-214) if any of those years would have been among your highest 35 years of wage-indexed earnings used to calculate your benefit rate. If that was done in your case you can likely rest assured that you're receiving full credits for your military time.
However, Social Security wouldn't have needed proof of your service if your earnings in the years that you were in the military prior to 1968 wouldn't have been among your highest 35 wage-indexed earnings years even with DMWs included. DMWs only increase your benefit rate if the years they're credited for are among your 35 highest years of wage-indexed earnings. So, you're probably receiving the correct benefit rate even if you weren't asked to submit proof of your service. But, if you have reason to believe that you didn't receive proper credit for your service you could submit proof of your active duty to Social Security along with a written request for recalculation of your benefit rate.