Originally Posted by NOISEpollution
Probably should've looked first.
How can I be sure that I did everything correctly? I don't want to screw something up and ruin an entire batch by pitching bad yeast.
So the truth is, you can never be absolutely sure. But you can do some common sense things. Cacaman suggested making a starter, which is a great idea. I actually taste a little of my repitch starters just to make sure i don't taste or smell anything funky going on. Don't re-pitch if you have reason to believe the fermentation the yeast came from was suspect.
The downside of repitching is that you're also propagating any bacteria or wild yeast that made it into your first batch, but if you're careful and practice good sanitation, there's no reason you can't successfully repitch for multiple generations.
One thing to keep in mind, as you continue to repitch yeast, getting the yeast adequate oxygen is more important than on the initial pitch. When you buy your yeast, they're all stocked up on their sterols (which are necessary for reproduction and good healthy cell walls), but that can be depleted during fermentation. If you don't give repitched yeast enough O2 to rebuild their sterol supplies, you can end up with problems.