In Visio, set the Database | Options | Document | General | Symbol Set to IDE1FX instead of Relational. The change in Symbol Set will cause the diagram to show Cardinality (one, many, optional, etc) using IDE1FX notation. You can also set Database | Options | Document | Relationship | Show options to use either Crow's feet or explicit cardinality. Visio is good for simple schemas, and is a great tool for reverse engineering specific (very small) parts of a database for project specifications.
I still prefer ErWin, especially for large or complex jobs.
-PatP