When can I use an ovulation test?

Stacie • 31 and carrying an Autumn baby
Hi guys, glow changed my fertile window after I logged the other day so I'm in a bit of a quandary now. I've bought an ovulation kit to help out pin pointing. I've read you can do it just before you ovulate, does anyone know if it detects when you're actually ovulating? 
​As in if I think I'm ovulating today will it give me a positive or negative? Or does it only detect before? 
​Hope that makes sense. 
​Thanks :)