Bi-directional DShot can help you here. After updating the BLHeli firmware on your BLHeli_32 ESCs to the latest version (support was added only recently) and the firmware on your flight controller to Betaflight >4.0 (PR on Github), you can go into the Betaflight Configurator app and enable Bi-directional DShot under the
It must be said that the easy BLHeli ESC firmware upgrade is only available for BLHeli_32 ESCs. There are ways to get support for bi-directional DShot on BLHeli_S ESCs, but they're more complicated. Instructions are linked to on the Betaflight Wiki page.
Motors tab in Betaflight Configurator
The ESC-reported motor RPM values will then be displayed under the
R label next to each motor in the
There is also an OSD widget for viewing motor RPMs, called
OSD_ESC_RPM in the code, which can be enabled under the OSD tab.
As @BenWilson points out in his comment on this answer, you can also set the BlackBox debug mode to
DSHOT_RPM_TELEMETRY and get motor RPM values recorded in your BlackBox log The implementation is described on the Betaflight Wiki here.