Usually a fish store will test them for you. I would take a sample of water from the tank and a separate one from the tap, that way you can see if there is much difference. pH will usually be different after 24 to 48 hours than right from the tap.
Discus are an advanced to experienced aquarist and are special needs/care/habitat fish to do them justice. Locally raised fish would be a best option as raised in similar water parameters. There is a HUGE difference in properly raised Discus and unknowingly or neglected/poorly raised/cared for fish, even just visually.
Personally I would never recommend them for the average keeper and I LOVE Discus. You kinda have to be in it like 1000%, (yes thousand %). They require much special attention be be excellent like they should be.
As for Rams, the more specially bred/fancy they are the more delicate they are likely to be. Again, home raised and especially locally raised fish would be the best options.
Bolivian Rams would be the least sensitive/delicate - most hardy, from what I understand.
Check into an Aquarium Society meet up near you. Best condensed group of fish loving people and connections to learn and meet hobbyists loving the same kinds of fish you like.