What cards? What features?
Safety class digital inputs generally monitor to make sure that the proper Tx pulse train is received at the input.
Digital outputs can have electronic trips to shut down a channel on output overload. They can also have open circuit detection, generally by putting a low potential or pulsed current out to see if there is an actual load.
Analog inputs like 4-20mA inputs have a live zero detection, and out of range detection.
Analog outputs can also use a low-potential or pulsed waveform to determine if there is an actual load present, and can monitor for overload conditions.
Thermocouple/RTD inputs can tell if there is an open circuit, or short for RTD's.
Encoder input modules, depending on configuration can detect open channels by not seeing a complementary pulse on the opposite channel, or on the NOT channel for differntial inputs.