A quick google search seems to indicate that the test is to find calcium (carbonate etc) in rocks. Here is the deal though, this should be a good thing for frogs (having a bit of calcium in the water) UNLESS the calcium is strong enough as to dramatically change the PH of the water, and slowly poison the frogs. If you are worried about it, get a PH test kit for pools, and see where you are at with your water bowl water. I'm assuming you change your water bowl water frequently, an as such, I don't think you will run into an issue. But if you are the type to leave it in there for weeks (bad thing to do), check the Ph every day over time and see if it climbs, this will tell you if your rocks are leeching minerals. Again I want to stress this is not necessarily a bad thing, if it were me I would purposefully add reactive rocks to give a tiny calcium boost, but prolonged soaking might cause problems if it was the right type of rock (like chalk, which is pretty much pure calcium carbonate). Even then I'm not sure it would hurt, as it should reach an ionic equilibrium, but there do exists toxic alkaline pools in nature, so the correct minerals are out there somewhere. Where did you collect the rocks? If they are from your backyard, and were not surrounded by death, I suspect they will be fine, just change your water daily... I would also look into the mineral names themselves if you purchased the rocks, a vinegar test won't detect the mercury in cinnabar, etc.