You check the impedance of the integrated sound transducers from the connection plug. The impedance is sometimes hot-embossed onto the plastic shaft of the plug:
If there is nothing there, the model has an impedance of 250 ohms.
If it reads 32 Ω, the model has an impedance of 32 ohms.
If it reads 600 Ω, the model has an impedance of 600 ohms.