SQL Server 2005-2008 Compatibility Check
Is the 2005 instance still your production instance? If so, you could use a server side trace to capture a workload and replay it on the 2008 instance. You should try to capture your normal workloads. You have to decided what is normal in your system. This wouldn't guarantee that all stored procs will work, but you day to day activity should be fine.

As far as compatibility levels please review the BOL link:
http://msdn.microsoft.com/en-us/library/bb510680.aspx