For the longest I used the General Hydroponics PH test kit where you put the water in a vial and drop in some of the solution and look at the color chart. But I'm a little color blind so that wasn't all that accurate for me.
I bought this $14 Pen on Amazon and I've only had it for a couple weeks but it's super convenient. It's nice I still have the PH Down/Up from the GH kit though.