Where do I find how much of a gross distribution on a 1099-R was a RMD?

"Generally", if the 1099-R was from an annuity or pension, the payment is set so that it is the RMD. So 100%.

If the 1099-R is from an IRA or similar account, then your IRA custodian will be able to tell you what your RMD is (note: they may need to know the sum of all your IRAs if you have more IRAs than with that one custodian).

And actually for 2020 it doesn't matter.  The RMD was waived for 2020 and not required.  So it's ok to just say NO, the RMD was not required and it is not the RMD.