An average genuine 18650 batteries will weigh about 45g and no nothing less than 42g. You can see I have a fake 18650 battery in the pic weigh 32g only and the genuine 18650 battery weight 45g. Some genuine 18650 battery can weigh more than 50g. Good brands will tell you the weight of the battery.
You can use a digital kitchen scale.
Why fake 18650 battery weigh less?
That is because inside is a smaller battery with step up circuit which wrap in layers of paper and stuffed inside a 18650 battery size case then heat shrink in a tight plastic label sheet to make it look real.
Genuine sellers will specify battery weight in their product description/sheet.