You can check this value from:
Settings: Go to Settings > Phone information > Software information > Build number. The SW REV value is usually the 5th character from the right.
Download Mode: You can see the RP SW REV value. The main SW REV is BX, where X is the SW REV value.
Recovery Mode: In recovery mode, you can see the AP version and check the SW REV value similar to the Setting’s AP version.