You *could* test for ammonia and/or nitrate . . . but I don't see the point.

The solution is the same, regardless: if you add chlorine in the late PM, and it's gone by the early AM, then you need to add a lot more chlorine that same PM, regardless of whether the 'goo' as CYA leftovers, PHMB leftovers, or algae.

In other words, since the test isn't going to change* what you do, why bother?

Ben

*There is a bit of an exception. If it's ammonia or urea, keeping the pH high is best. But, if it's algae or Baquacil, low is probably better. However, the 'differential diagnosis' turns on the CYA level now, vs last fall. If it dropped a bunch, you should assume that the pH needs to be at the high end.