Is there a way that can help us to check whether our site being hacked or not? Please let me know if there is. Thanks
Thats a pretty broad question. Do you think anything weird going on ? You could start by comparing your files with a backup copy and see which files are changed
As John said, that's a loaded question. Generally though, your site isn't "being" hacked. Its not generally an ongoing state, its either been hacked or it hasn't. As asked before... what makes you think about this issue? Did you notice strange things going on? Do you have a dedicated server? Hosted? Do you have admin access to it? Does someone else admin? The comparison of your backup to what's there is good... only if your backup was made before your site was compromised, otherwise you're going to be comparing hacked files in your backup to hacked files in your production site. Post back with more information and we can help you some more.
You can ask your web host or server administrator to check access logs and FTP logs and see if any unknown IPs are listed; ones that do not belong to you or individuals you've provided access to.