By measuring it of course. All you need are a couple of graduated containers, a pyrex pitcher-type deal for measuring the progress of starters, and a rising container or two for the finished doughs (only not that big…gallon-sized is plenty). You can find them at any restaurant supply shop in pretty much all sizes. I’ve got quite a few of them, since they’re great for measuring fruit for jam-making, brining chickens, you name it. You just mark the side of the container with a piece of tape or grease pencil and set your timer. You’ll be able to tell which college your starter is going to in a few hours’ time.