Hey! I scan the entire contents of the table. I have a cache layer in memory anyway so that works quickly. If you have multiple types in the same column, it just treats the type as "object". If you connect, and start entering various types into your columns, you'll notice in the object explorer that it updates the type information.
I have used excel as a data source using VBA before so one problem I had there that I'm curious how you approached was how to automatically determine data types? IIRC the ODBC engine in VBA scans a set amount of rows to guess. Are you doing anything different?