We test in 2 main stages here at Cubeb.
The Stage 1 testing is when the website is finished and gets reviewed by team members who have not been involved in the development of the project. (fresh eyes!)
These users create a list of some obvious issues in the system and make suggestions about usability of the site. This list is then addressed by the design team and the project moves to Stage 2.
Stage 2 involves releasing the website in a live "test" for our client to run through all aspects of the new site and make their own list of issues. Any issues are then addressed by our design team. This gives our client the opportunity to test-run the site before we release it to the public at large.
Once stage 2 has been completed and signed off, the site is put into deployment.