Are there chances for extension to be not safe? If yes,then how to check if extension is safe or is it already checked by the server.
In my opinion, all extensions are safe, because if it is not safe or damaged you can delete it from your project.
Please further explain what you mean by safe?
Means, Does any extension have chance of having a virus or something.
If yes then how to scan it or how to know if its safe?
see my answer here