Are your customers given recommended instructions for proofing, baking, etc?
I am unaware of a quantitative test that will work, but you could try these qualitative tests without much cost.
Start by pulling a few pastries that have been prepared and packed as your customer will receive them.
Prepare them following the provided instructions precisely.
Take a picture of results and keep in your records.
You might also consider retaining some frozen pastries to test in the future. This would show if your yeast is being killed by freezing.
If a customer complains you can find and prepare the pastry made in their batch.
The sampling frequency would be up to you. Maybe once a batch, once a week, or once per container of yeast?
If every pastry you bake is flawless, then the issue is probably not on your end. Plus, what matters most is the effectiveness of the product. You'll never have a customer complain of 200 (units?) of yeast vs 250 units of yeast. They will complain if it does not rise.
Either way, I would suggest asking customers for more details. Did they use a proofing oven? What temperature were they baked at? Is the customer using a metal sheet pan, silicone bake mat? etc
In my experience, I feel like these variables are more likely to blame than an inconsistency in your yeast.