If you scroll down to page 3 of the 1099-R Wks you can track the basis recovery each year.  You'll need historical data on when the retiree started to compute the amount of basis recovered to date.

See Pub 721 for details on how it works:

https://www.irs.gov/pub/irs-pdf/p721.pdf

At this point OPM does this 99% of the time and properly reduces the taxable amount in box 2a.  Back when I started I saw a lot of box 2a "UNKNOWN" entries.