Attempting to make yeast starter from older yeast

I am trying to make a yeast starter from a WL vial of Kolsch yeast that had a use best by date posted in June in order to get one more brew session before the cold arrives (cannot run my wort chiller in the winter). I made 750 ml or 1.030 wort and it has been on the stir plate for the last 24 hours.

1. Any idea of when I should try adding fresh wort?
2. What physical signs should i be looking for to see if the yeast is still viable?

I'd put my money on your yeast being alive and well. The best way to know if you've still got lively yeast would be to take a gravity reading. I would give that starter a couple of days, put it in the fridge overnight, decant the liquid, and add another liter or so for another starter. You'll have plenty of yeast by then. Don't rush it, you've procrastinated since spring. What's a few more days?

I am hoping to be able to brew next weekend with just one or two build up steps. I'll let it go through Sunday before placing it in the fridge overnight and adding fresh wort.