Hi Paul. Not sure if it’s what you want, but you can click on the S-meter and it will turn into a centre zero meter. I don’t think it actually quantifies the error, but neither did the ones on the old analogue radios.
There’s also the synchronous AM mode which (I think) gives an error reading. I haven’t used my RSP2 or Uno for a little while so forgive me if I’m a bit rusty!