Next to the heat sink on the secondary side, you will also see many capacitors and inductance coils, which together form a low-voltage filter module – if you find them, you will also find the secondary side.
An easier way to tell the difference between the primary and the secondary is to follow the power lines. Generally speaking, the secondary side is often connected to the output line, and the primary side (input line connected from the mains) is connected to the input line.
After the mains is connected to the PC switching power supply, it first enters the transient filtering circuit (Transient Filtering), which is what we often call the EMI circuit.