The cheapest way would be to send a swab out to RAL (
https://www.vetdna.com/). That test is $20 (plus sampling supplies).
A vet could diagnose with a fecal sample (not always available in non-feeding animals, though). A visit and fecal at my vet is a little over $100.
On the Metronidazole discussion: I have used it to treat snakes (and a turtle) that aren't feeding, and that sometimes does the trick (which implies that the non-feeding was caused by amoeba in those cases). I personally would not shotgun it in an animal with no symptoms. It is also pretty far down the list of issues I consider first in a non-feeding animal -- enclosure setup/parameters; prey item undesirability; stomatitis/"mouthrot", at least)
One problem with treating failure to feed with shotgun meds is that it can be a little slower than a vet check, and only rules out one issue among many. A vet can rule out a whole handful of things and often get the right treatment going on the day of the visit.
On broader issues: one way that I've found pretty reliable to avoid issues like this one is to be incredibly selective about who you'll buy from and what you buy. Taking a free animal is something I would not do (I assume a seller knows what their product is worth...), nor "rescues" or deals too good to pass up.
The ball python market is absolutely saturated to the gills; it is 100% a buyer's market. There are many hundreds of breeders to choose from, and tens of thousands of snakes. A buyer can take the time to find an absolutely solid snake from a very high quality breeder that hopefully pathogen tests their stock and still pay a fair price.
On QT: You might consider improving your procedures since you have a few snakes and some intent to get a breeding collection going. Amoeba probably won't take a collection out, but there are viruses that will and it is worth protecting against them.