Originally Posted by BendBrewer
How cool must it be before adding the yeast?
It depends on the strain of yeast but all strains would have temp ranges on the packages. 60-70 degrees in a normal range for ale yeasts.
The rest is great information, but I have a small disagreement with this. Everything I've read has supported the idea of pitching (ale) yeast at slightly warmer temperatures. 75 degrees is usually what I aim for. The range on the package is the fermentation temp, which should indeed be between 60-70 degrees for most ale yeasts.
How can you tell if your yeast is still active, before using it?
BendBrewer's advice is spot on for liquid yeast, but for dry yeast a starter is not necessary. The second pouch of yeast is probably cheaper than the DME you'd put in the starter. However, to make sure it's still active you can rehydrate it shortly before pitching and add sugar to proof it. Check the link below for more details.