Question about Age of Yeast and Viability?

Is there a fairly accurate way to predict the viability of a package of yeast (both Dry and Liquid) based on its age? (Assuming that the yeast has been kept refrigerated the entire time.)

The initial impetus for my question is that I have a packet of Safale US-05 and Nottingham that have been kept in my fridge for over 9 months. I have no idea if they are still viable and, if so, at what percent. Does it help to keep Dry yeast refigerated? My interest in this subject applies equally to Dry and Liquid yeasts.

Thanks!

I've used liquid yeast that has been sitting in the fridge for over a year w/o issue. It took a bit longer to swell, but once it did, it fermented to completion w/o any problems. I would make a starter with that yeast to see what goes on and go from there.