You can't just take the checkride. There has to be a record of ground and flight training and you must have an instructor' signoff for the checkride. See 61.157(b). Most training centers have a short course of a few days of ground school and a few sims to get your ready. Plus to use your ATP to skip the FAA private and commercial your CAA has to validate your license to the FAA. And there is the TSA check.