How can you tell when a starter is ready when it.s on a stil

I made a starter from a smack pack last night at 8 pm. i want to brew on sat. how can i tell when the stir plate is on if its ready? the smack pack is wyeast 1056 for dead ringer brew.

I go by when the foam has receeded. If its all foamy and Krausen(y) its still going. YOu’ll probably be fine if you shut it off tomorrow morning, throw it in the fridge, and decant a little bit of the beer on top before you pitch.

If I don’t plan to decant, I actually like to pitch at high krausen. Makes it really take off strong and fast.