Have you had a delay in your menstrual cycle like this before? Are you periods regular or irregular? Were you taking birth control before? I think that the lack of a menstrual period is a concern that you need to look into. Especially if there is a pattern...it might be caused by ovarian cysts or a hormonal imbalance...or something else.

Generally, the earliest time that you can test is 4 days before your period is due to arrive, but since you don't know when that will be, you can test maybe in a week, or so.