I have a couple of old containers of B.T. from previous years. Does anyone know if I can use them or do they have a shelf life? There is no info on the containers.

Toss them. Powedered Bt has a shelf life of 12 to 18 months and the liquid form is about 6 months.

B.T. is a live organism (though dormant in your product). It all depends on how well you store them, shelf life can be as long as 3 years but overall effectiveness could be from 25 to 75%. High or freezing temperatures will markedly shorten shelf life as will open containers subject to the elements. The only way to test would be to either do a lab test or check the viability on a test pest.