I wish there was a way to "proof" cultures like you proof bread yeast.
This isn't exactly "proofing" a culture in the context above but there is a very good, accurate and reproducible way to check on the virility of any starter but you need a pH meter (and buffer solutions to calibrate it accurately). Here is the basic idea:
Take a quart of any standard milk (P&H is fine) and hold it at the starter ripening temperature.
Take the pH of the milk at the above temperature.
Add 1/4 level teaspoon of a freshly opened packet of culture.
Sprinkle it on top and let sit for five minutes.
Stir it in for one minute.
Measure the pH at 1 hour, 2 hours, 3 hours, 4 hours, etc for as long as it takes to generate a decent curve of the pH drop versus time. The first time I'd take the pH at 1/2 hour and 1.5 hours in case the pH drop really takes off.
When you draw a graph of pH versus time you have a standard for a fresh sample of the starter in question.
At some future date repeat this for the same bag of culture and compare the two graphs.
This will give you an excellent idea of the viability of the culture.