Ovulation perdictor kits only PREDICT when you SHOULD ovulate...they don't verify that you actually did...so thats the tricky thing with those.
One way to know that you ovulated...which is what I do...is use the predictor kits and take your BBT every morning. When you ovulate your normal BBT raises between .3 and .6 degrees and remains high, this is because when you ovulate your body releases progestrone which is a heat-emitting hormone, thus causing your temp to rise...and it stays high for about 13 days, after that it will return back to normal if the egg was not fertilized and implanted. If your temp stays elevated for 18 days it is almost certain that you are pregnant, and you temp will remain high for the duration of the pregnancy.
So....18 days post ovulating (if you did in fact release an egg) you should be able to get a result on a HPT test. So right now it would be too early. But keep in mind every woman is different, and hormones levels are different. HcG dubles every two days, so if you really think you may be pregnant...test on the 19th, if its negative, wait 2 days and test again...and you could always go to the doctor for a blood test.
Hope that helped a bit

Good luck!