Do you refer to frame errors?. The 8051 manual says the serial port behaves in this manner (not a literal transcription):

1) If the received signal does not remain at '0' half a bit time after the start bit leading edge (at the middle of the start bit) the start is not considered valid, so the receiver waits for a new start bit leading edge. No flag is set.

2) Every bit is sampled 3 times, the value found in two or more samples is considered valid.

3) If the stop bit is not valid, i.e. if it is a '0', the frame is not accepted, RI is not activated, and the receiver waits for a new start bit.

As you can see, the 8051 uses only simple mechanisms to determine frame errors, and it doesn´t set any flag to inform you. Of course, RI is activated only when the receiver thinks it has a valid frame for you.

I must say I prefer this schemma to the more powerful, but complicated, error detection capabilities found in the PC's UART.

Alfredo.