By clicking the "Toggle Schema Explorer" button you can view all the table sources available for use. They appear like a tree, and by selecting the specific source table you can view all the table columns.
You can immediately generate a select statement with all the columns if you click the generate query button when you hover to a specific table. You can start working right away with just one click.
Other then that you can view all the columns of a table by checking the table:
Or by clicking the table icon you can preview the table: