1. Appearance observation method:

　　The number of layers of a PCB board can be preliminarily determined by observing its appearance with naked eyes. Under naked eye observation, multi-layer PCB boards appear thicker, while single-layer or double-layer PCB boards appear thinner. However, due to the trend of miniaturization in modern electronic products, many PCB boards are very thin, making it difficult to determine the number of layers solely based on their appearance.

　　2. Line density method:

　　By observing the circuit density on the PCB board, the number of layers can be determined. In general, the circuits on multi-layer PCB boards are denser and more complex between circuits. By comparing with empirical data, the number of layers of PCB boards can be preliminarily determined.

　　3. Cross hair method:

　　This is a commonly used judgment method that requires the use of a microscope and a crosshair ruler. Using a microscope to observe the edges of a PCB board and measuring the thickness with a crosshair ruler, the number of layers of the PCB board can be accurately determined by comparing the measurement results with empirical data.

　　4. X-ray method:

　　This is a professional testing method that requires the use of X-ray instruments. Through X-ray scanning and analysis, the internal structure of the PCB board can be visually observed, thereby accurately determining its number of layers. However, this method is more expensive and impractical for ordinary users or small businesses.