What are the qualities of being a good web developer?

Look for references and not just URLs with their work (because an unscrupulous person could point to anything). Also, try giving them a small task where it won't matter much if they screw it up, and see how they handle it.

If the developer has given you their portfolio then by conducting small practical exam based on their portfolio you can identify developer and his/her work easily.

Because through the theoretical exam, you can only estimate their knowledge about how things are going to work but how to do it you will not estimate.