How to check yeast starter activity

Hi guys,

I'm trying to reuse my yeast from a previous batch. I washed it, stored it for a week in the refrigerator, made a starter as explained on many website, pitch my washed yeast into it.

For the details, it is in a 500 ml glass vial with a soft plastic cap that allow CO2 to get out a little.

BUT ... yes but how can i see if my yeast is active ? what are the signe ine the vial that let me know that I will be able to use it in my next batch tomorow ?

Are you using a stirplate or intermittent shaking? It should be evolving off CO2 which with shaking you'll see happen as foaming. I just recently started using a stirplate, and i dont get the foaming action, but it does create a little mini-krausen around the edges.