At the end of the cycle, you should have 0 ammonia and nitrites. Nitrates will be there (because that's what those first two are converted to). Do a decent sized water change to bring those nitrates down and you should be good to add a fish. Don't add several, be patient. If you add too many fish at any time, it will overwhelm the beneficial bacteria and the ammonia/nitrites will start to register again.
To really be sure, you can get pure ammonia and dose that up to 4ppm. In a cycled tank, that should drop back to zero in 24hrs (or close to that).
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