Short the inputs with a copper wire (U shape) and put it on 100 NPLC / 10V range / NDIG 9. The last digit should be stable, single digit moves around the zero.
If the unit has a drifty ADC, you are in for $800 - $1400 depending on what Keysight has in stock.
If the unit has changed hands many times, your risk of a bad ADC is higher. Bad ADCs can appear in pretty much any unit, but maybe post-2008 are better.
In any case, these units are expensive to get now, and you can live with the drift.