You can preview what your data will look like via the Preview A and Preview B options in the mini-toolbar.
- Preview applies the Data Type conversions in the Schema Map.
- Preview A - Returns the Data from the Source including any Dynamic/Calculated Columns, Lookups and Filter Applied.
- Preview B - Returns the Data from the Target.
If you have a problem with a Data Type conversion, change the Data Type to System.String and then you can preview the Data to see what the problem is.