The typical days are 2-6, 3-7, or 5-9. Some take it 1-5 or 4-8 or some other variation at the beginning of a cycle. I think the days a doctor tells you is mostly based on preference, but I've also heard that the earlier you take it- the more eggs you will potentially ovulate and ifyou take it later (5-9) you will most likely only have one more mature egg.
To test for PCOS they may use a combination of bloodwork and an ultrasound. They test FSH, LH, testosterone, and will usually also test fasting glucose and insulin (a lot of PCOS women are insulin resistant), as well as prolactin and TSH to rule out problems with that.