I have no idea how to test reliably, and I am not asking about a test by myself, I am simply asking for the ultimate answer to my question:
The maximum board-specific Arduino UART clock has to be quite close to one of the following (it's limited to this list because the corresponding Raspi supports just this subset > 115200):
230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000 and 4000000
the different Arduino boards are the following:
16MHz AVR (e.g., Nano, Uno or Mega2560) <=> Raspi,
Zero (M0, 48 MHz cpu) <=> Raspi,
Due (M3, 84 MHz cpu) <=> Raspi,
and esp8266 (80 MHz cpu) <=> Raspi
Which max UART clock values exactly for either Arduino board has to be chosen?
IIUC, 500000 has been suggested at max for 16MHz AVR boards (or maybe perhaps even 1000000?),
but which is true for the rest of the boards and MCUs?