How do I know, or find out if I am receiving extra income from Social Security for serving in the US Army in 1970_1971
The extra income you're referring to are deemed military wages (DMW). During the time you were in the service, Social Security credited you with an extra $300 in wages over and above your actual wages for each calendar quarter (e.g. Jan-Mar, etc.) in which you had active duty (https://www.ssa.gov/OP_Home/handbook/handbook.09/handbook-0953.html).
Starting with 1968, Social Security automated the process of crediting DMWs based on earnings reports received from the various military service branches. During my career with Social Security, I found this process to be extremely reliable, so I am confident that you received your proper credits. To answer your question, though, the best way that I can think of to verify that you received the credits is to compare your W-2 earnings from your years with active duty with the amount posted to your Social Security earnings record. Your earnings record for those years should reflect the extra deemed wages.