ChopSuwe is right but to clarify, in many appliances, the low voltages to drive logic and LEDs is derived directly from the mains supply and may be at or nearly at live mains voltage. What that means is the circuits themselves may operate on low voltage but those low volts are not isolated and contact with a live, low voltage circuit can expose you to mains voltage to earth.
With the right knowledge and expertise, optical couplers can safely be used to connect to such a circuit, but there are usually other ways to read the status of the appliance. There’s not necessarily one way as appliances vary so much in their design and implementation. Many “intelligent” appliances (as in “having a microcontroller”) will have some sort of bus available for control and status. Such buses are usually one of a small number of common bus types.
Simply put, you need to get to know the appliance itself if you intend to hack it. That’s always the case. A hacker needs to learn a lot about the target system.