The red square mark of 20 can be used to identify authenticity in the following ways:

1. Test the flexibility of the cigarette. Real cigarettes have good flexibility and will not break even if they are bent, while fake cigarettes are not as flexible as real cigarettes and break easily.

2. Check the cigarette brand. The trademarks of real cigarettes are printed clearly and the patterns are vivid and lifelike, while the trademarks of fake cigarettes are printed roughly and the patterns are blurry.

3. Check the tobacco leaves of cigarettes. The tobacco leaves of real cigarettes have a mildew and pungent smell, while the tobacco leaves of fake cigarettes have no mildew and pungent smell.

4. Check the cigarette carton packaging. The packaging of genuine cigarette rod boxes has good sealing and neat edges, while the packaging of fake cigarette rod boxes has poor sealing and uneven edges.

5. Check the plastic film packaging of cigarettes. The plastic film packaging of real cigarettes is smooth and soft, while the plastic film packaging of fake cigarettes is rough and clumpy.

6. Check the cigarette smoke. The mouth decoration printing on real cigarettes is clear and the pattern is consistent, while the mouth decoration printing on fake cigarettes is blurry and the pattern is inconsistent.

7. Check the tinfoil packaging of cigarettes. The tin foil packaging of real cigarettes has good sealing and neat edges, while the tin foil packaging of fake cigarettes has poor sealing and uneven edges.

In general, the authenticity of the 20 red square seal can be identified through the above methods. If the red square printed product you purchased meets the above characteristics in the above aspects, you can initially judge that what you purchased is genuine.