Ovulation Tests?

How do ovulation tests work ladies? I mean when do you start taking them to see if you're ovulating? Since going off the pill my period only lasts 4 days so I wonder if the app is off. My period usually returns 1-2 days before what the app predicts. I finished my last period 2 days ago so how many more days before I should start using the ovulation tests? I had NO issues getting pregnant with my 1st but its our 4th month of trying for our second so I thought I would give this a shot. Thanks!!