The PetSmart shop chain certainly do free water tests, and most aquarium shops will also test a water sample for you if you ask. You won't need much (50ml?) but try and take it fresh and also in a clean sealed container with as little air space above the water as possible (ammonia is volatile). And if they use the Tetra test sticks (a little stick that says it tests everything) bear in mind that these can be very inaccurate for nitrite (I've switched back to using liquid test kits).

Good luck!