Dear experts

I am new to this forum, anyone here please help me to understand what kind issue may have when we upgrade office from 2003 to 2013?

We are planning to upgrade office 2003 directly to 2013, before upgrading, we would like to understand :

1. how to identify which file(esp. Excel files) have macro inside?
2. any potential issue may have after upgrading?

Thanks in advance for any input/thread on above !!