How to quickly distinguish whether the vacuum flask is insulated?

Time：2020-10-23 20:33:00

After adding boiling water to the thermos for two minutes, touch the lower part of the cup to cool, the lid is hot, and the insulation is normal. If the lower half of the cup is found to be warm, it indicates that there is a problem with the vacuum of the thermos, so as to determine whether the thermos cup Insulation.