In the oldie days, I would start a tank with everything running except no fish. I'd put food in the tank to cover the needs of a few 5 inch fish every day for 5 or so weeks and run the tank at 80-82 F.
I then added fish (juveniles) all in the 1-2 inch range, maybe a dozen. I did it that way with half a dozen tanks and never had a problem. Also, my tanks were always understocked. Maybe I was lucky.
Today, I'd probably use straight ammonia as it's cleaner and more predictable, or use a starter kit if some mature media wasn't handy.
Test kits are valuable for checking on a lot of things after you have fish, although their use is a bit less valuable IME (if you run tanks the way mentioned), if you keep a solid WC schedule and under stock your tank. That doesn't mean that test kits aren't good to have and use, but you have to be more conservative on water quality if you don't test (unless you are really experienced.) I'd still always keep a test kit handy just because it's pretty inexpensive compared to a fish.
Test kits are invaluable whenever you suspect something is wrong. Testing and determining right away can literally save fish, so it's just a good thing to have.