The basic approach is to record the behavior of your queries in 2008. Get the execution times, resources used, and, for really interesting queries, the execution plan. Then do the same thing in 2014. Just be sure you change the compatibility level on the database so that you see all the new functionality in use in 2014.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning