pool test strips. it's a little strip of paper that you dip in your water. it has squares on it that will change colors. compare them to the chart on the bottle and you are done. you can get them at pool supply places or any place that sells fish tanks. pretty common item.
or i got this one at a hydro place and I also used to work in a pool place and they taught us all about water so I can help there . I had to like take a t4st and shit for all this. look at the pic...........