Don't have a clue (not a Phissh Purson) but here's a suggestion:

Take a "sample" to either a Swimming Pool Dealer
or a Water Softener Dealer -

BOTH do "water testing" - usually right there "as you watch"...

Might be best NOT to mention "what" they're testing...
You could "mention" that if a "spoon" sits in the water for a while,
it turns black... but kinda hard to say your plants turn
black - not many folks leave their house plants *underwater*
for days at a time!

You need to deliver the sample in an UN-contaminated container -
most places will give you a lil plastic bottle to carry the
test sample in.

/s/ Phred