How do I check MovementMode from C++?

You can reference the character movement component using

GetCharacterMovement()

From there, you can either use the EMovementMode Enum, or the bool vars, the same way you would in BP’s.

void AMyCharacter::GetCharacterState()
{
	// option 1: use the MovemenmtMode enum
	if (GetCharacterMovement()->MovementMode == EMovementMode::MOVE_Swimming)
	{
		// is swimming
	}

	// option 2: reference the bool vars
    if (GetCharacterMovement()->IsSwimming())
	{
		// player is swimming
	}
}