The western toilet is designed so that it flushes once in a continuous flow after a person uses it. This ensures that any excrement is washed away. However from a fiqhi perspective it is known that for urine a object must be washed twice with a break(water stops flowing) in between washing. If one flushes a toilet which only leaves one continous flow is the water after the flushing considered najis?
If it is connected with a main line of water, and washes away all of the najasa, where you don’t see anything it should be considered pure. However, usually there may be some najasa on the side of the toilet that does not get flushed away.