If you can put an airlock on the starter to check if the starter is active, that'd confirm.
A single piece of cling wrap sealed loosely and sealed with a rubber band would also confirm as it fills. It shouldn't take too long, from a couple minutes to less than an hour to see a change.
Aroma also will tell you it's working. If it smells like green apples, then it's mid ferment and ready to pitch.