Hi Sunny,
Welcome to PakPassion, hope you enjoy your stay here.
There are few authenticity factors or each brand. Some batmakers stamp the shoulders, put certain stickers which can not be obtained from the market and so on.
When someone is selling a fake bat he usually uses high end bats stickers and stick them on a cheap quality bat to make most out of it. For example, some people will buy a Grade 3 willow bay and stick grade 1 stickers on them.
Sometimes the stickers also tell you that's it's a fake bay etc.
Tell me which bats are you talking about and people around this topic will help you out.