To distinguish between fiber felt and mixed felt, you can observe and judge from the following aspects:

1. **Material composition**:

- **Fiber felt**: Usually made of a single type of fiber (such as wool, polyester, etc.), the material is uniform.

- **Mixed felt**: Made of a mixture of two or more different types of fibers, which may include natural fibers and synthetic fibers.

2. **Appearance and feel**:

- **Fiber felt**: The appearance is relatively consistent, the feel is soft, and it is usually thicker.

- **Mixed felt**: It may have different textures or colors in appearance, and the combination of different fibers may be felt in the feel.

3. **Performance characteristics**:

- **Fiber felt**: Generally has good thermal insulation and breathability, suitable for specific warmth or sound insulation purposes.

- **Mixed felt**: The performance may be more diverse, combining the advantages of different materials, and may perform better in strength, wear resistance, etc.

4. **Burning test**:

- A small-scale burning test can be carried out. Different types of fibers will produce different odors and ashes when burned. For example, wool will have a burning smell when it burns, while synthetic fibers may produce a chemical smell.

5. **Labels and instructions**:

- Check the product label or instructions, which usually indicate the material composition to help identify it.

The above method can more accurately distinguish between fiber felt and mixed felt.