Is there a programming way to differentiate RM126x variant if I develop my own firmware?

The RM126x is based on the SX126x Lora radio from Semtech. Unfortunately, they didn't include any internal change between the SX1261 and SX1262 that would allow to differentiate them programmatically.

It is hence not possible to know the difference between the RM1261 and RM1262 by software only.

A workaround could be to leverage your own design board and add a pull up / pull down resistor on one of the spare GPIOs to indicate which variant it is.