Methods and apparatus for estimating received error rates. In one embodiment, the estimation of received error rates is conducted in relation to a bus interface such as a high-speed High-Definition Multimedia Interface (HDMI) interface, and the method utilizes corrupted symbols that violate TMDS symbol rules, the corrupted symbols being easily detected and counted. In one exemplary implementation, a symbol error rate (SER) can be estimated from the number of detected invalid symbols. The SER can be used to diagnose the performance of the HDMI interface, and optionally as a basis for selecting or implementing corrective action(s).