1. Observe the surface of caviar: When using a hair dryer to heat caviar, tiny bubbles and discoloration will usually appear on the surface. When all the bubbles burst and the surface turns golden brown, it means the caviar has been used up.

2. Smell the caviar: Caviar will emit a special aroma after being heated. When the aroma becomes weak or disappears, it means that the caviar has been used up.

3. Observe the amount of caviar: If you know the amount of caviar before heating, you can judge whether it has been used up based on the change in the amount of caviar after heating.