Once you check Processes tab in Task Manager, you will find sql server process and there you can find out the User Name. And easily find out with which user its belong to.
But as you say your CPU usage goes upto 100%, it can be because of some queries in your sql server which are fetching lot of data or firing query against large data table.
So I will suggest you to find out that first and to do that you can go for SQL Server Profiler, which is a easy and perfect tool to know what is happening in our database.
Microsoft Sql Server -> Performance Tool -> SQL Server Profiler
In that click on File -> New Trace -> Connect to perticular database.
And you will see window where you can see all details about query which is currectly running and using how much CPU usage.
You will find columns like CPU, Reads, Write from which you can check, because of which query its getting problem.
We have faced same problem before some days and we solved it like this only.
Try it and try to change query according to your requirement.