You looking at it right now (Any SQL Editor outside of Qlik). The other way would be to use the Qlik Data Manager, which shows you the contents of an imported table immediately. Downside is, you also have to do all of your queries through the interface without using proper SQL as far as I know.
When developing I load one table or set of related tables at a time and check the contents. then when adding more I'll comment out the first bit or use a new file to load the next group of data, when happy with all then bring them together. If your datasets are large you can use the debug option to load a small portion of each table to make sure the linking works as expected.