I have an older packet of yeast that has been stored in the refridgerator past the best by date on the package. Is it still okay to use the yeast? Is there a way to test to make sure it's alive still?
With liquid yeast, you loose around 20% of viability each month after the manufacturing date indicated on the package.
If your yeast in 2 month old, it should still be 60% viable, thus you should be able to make a starter with it, in order to know how, check this link : https://www.brewersfriend.com/yeast-pitch-rate-and-starter-calculator/.
It will allow you to calculate your pitch rate taking into account the viability of your yeast.
Another method can be to take a sample, dilute it, die it with methylen blue and count the alive/dead cells ratio under microscope to compute viability. (https://www.whitelabs.com/beer/cell-counting-viability-testing)