Stir Plate question

[FrizzleFry]

I recently built a stir plate. I have a 2 liter flask I've been using for starters. My question is, what procedure do you all use to ensure that you have viable yeast, when using a stir plate? I ask this because I am on my second batch with it.
The first one was some WLP775 for cider. As expected there wasn't much in the way of krausen, just a little ring around the top. But after running it for a day you could see the yeast, which then thickened up nicely when I shut it down. Note that this starter was about half juice, half DME & water. I crashed and decanted, warmed and pitched it and it started fermenting vigorously in less than 8 hours.
The second one is some WLP500 for a Trappist Ale, but things look a bit different. Its been a day and there is no krausen at all, it doesn't really look any different. I am wondering if I should have waited to turn it on until I saw some activity. I am using some sanitized foil instead of an airlock, so I can't see any bubbling.
So how do I know if I got a good vial of yeast? And what should I do different in the future?

 

[TrainSafe]

When using high quality yeast like White Labs or Wyeast, they're way more likely to be viable than not. I've not heard of any yeast that is DOA (assuming you are within 6 months or so from production). That doesn't mean it can't happen, just that it is as rare as airplanes landing on the freeway.

Stir plates make it difficult to visually determine if the yeast are working or not. They stay suspended, and the CO2 is driven off continuously, so you can't see it.

There are really three reliable ways to determine if they're alive:
1. take a gravity reading with your hydrometer
2. put it in the fridge and inspect for the nice white layer the next day
3. pitch it in your beer and watch for signs of activity.

Think about it this way: before you knew enough to make starters, you just pitched the yeast directly out of the vial and hoped it was good. Starters don't change that dynamic at all, they just enhance the chances of you having a sufficient volume of cells.