If I have a 1099-R for selling stock from my 401K at 70 1/2, how do I show capital gains without receiving a 1099-B form?

We are over 70 1/2 and sold stock from our 401K that was reported on our 1099-R.  How do we go about reporting capital gains on that stock that was sold?  Is that necessary?  We only received the 1099-R and not a 1099-B from at all.