Ethernet has an "collision detect" function. By reading the bus at the same time as writing, and having dominant and recessive levels it is possible to detect an collision. Writing an 1 and reading an 0 is collision. This i done in the Media Access Circuit.
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The principle of superposition holds for waves on a transmission line. Ethernet (the old coaxial type) has such low attenuation that all points on the line have the same signal voltage. When two transmitters are on at the same time, their waveforms add up to a higher peak whenever the two signals are at their peak. This happens frequently during a pair of packets at the same time. By using an ordinary voltage comparator on the line you can detect this condition.
When an ethernet node transmits, it applies a DC bias current as well as the 10Mbits modulation current. Note I say current, let's assume it's 10mA into the 50 ohm impedance of the coax, so the voltage waveform on the coax is offset from zero by 500mV during one node transmitting. When two stations transmit simultaneously, the offset voltage is doubled (the currents add), which is very simple to detect.