How common are evap lines?

andrea
I wish I had a picture to show you all. But I threw the test away so it doesn't matter. I took a test the day before my period was due. It was not positive. I looked at it a couple minutes and threw it away. I walked away and came back about 15-20 minutes later. It looked positive. Now I'm wondering if it was positive or a false positive? Is there any way to know?