SQL Server Windows NT 64 bit Sudden High CPU Spike
There are a few ways to approach this.


Download sp_WhoIsActive and run it when you notice a CPU spike, or log it to a table and query the table looking at the CPU column (maybe ORDER BY CPU DESC).
Examine the plan cache using a free script -- I co-author one called sp_BlitzCache. By default, it will return the top 10 CPU consuming queries in your server's plan cache.
Buy a monitoring tool like SentryOne Performance Advisor and use it to examine your server during a spike.