Keep an eye on the colour of the yeast sediment. A healthy yeast is pale beige.
Also, if you're decanting off the spent wort between steps then taste it. It should taste like bland beer. If you detect sourness or sharp vinegar then it's gone bad.
Also, if you're decanting off the spent wort between steps then taste it. It should taste like bland beer. If you detect sourness or sharp vinegar then it's gone bad.