How can we find out whether my husband is getting credit for the 6 years he spent in the National Guard?
The only Social Security credits that your husband would receive for his time in the National Guard would be for the periods classified as active military duty. Most commonly, that's limited to a member's initial period of training for active duty, and their annual 2 week camps. During those periods of time your husband's military salary would have been subject to Social Security taxes, so those earnings would show up on his Social Security earnings history. Also, he would receive the appropriate amount of deemed military wages (DMW) for those periods of time (https://www.ssa.gov/OP_Home/handbook/handbook.09/handbook-0953.html).
All of the appropriate credits, including DMWs, would have automatically been posted to your husband's earnings record from 1968 to the present. For periods of active duty during National Guard service prior to 1968 Social Security usually needs proof of the active duty periods, such as a form NGB 22, in order to credit DMWs.
Based on my 36 years of experience working for Social Security, if all of your husband's active duty time was in 1968 or later he can rest assured that he's been given all of the Social Security credits to which he's entitled for his time in the National Guard. But, if you're not the trusting sort then about the only way that you could independently verify that your husband's service has been credited would be to obtain a detailed earnings query (DEQY) from Social Security. Social Security doesn't furnish DEQYs for free, though, so you would need to submit a request for the DEQY and pay for the time that it takes to them to produce the records. If you choose to do that, your husband would need to submit his request via a form SSA-7050 (https://www.ssa.gov/forms/ssa-7050.pdf).