You can test expressions when you configure expressions in the following ways:

In an output or variable port in the Expression transformation
In the Mapping Outputs view of an Expression transformation after adding the transformation to a mapping

For example, after configuring an expression that concatenates the first name, a space, and the last name, you can enter sample data for the ports, and then evaluate the expression to verify the result.

The following image shows the results of an expression that concatenates a sample first name and last name: