Lately the server load also comes in 6 / 7, there is a site that overloads the server.
How can I tell which site is it?
I hope to be understood
There isn't a specific way to determine how much server load each cPanel account is contributing directly, you would have to contact your hosting provider and request for that information.
However, the server load which you are seeing is contributed by all the other cPanel accounts on the server as a whole. Hence having a high server load might not necessarily be due to any of your websites under your reseller account.
As a rough guideline, the amount of load a server can handle is proportional to its number of cores - if your server has 16 cores (viewable from the Server Status page) while load is around 6-7, then it shouldn't be much of an issue.