of course people go to the States and dont declare and you should be okay, but once you contact the USA embassy they have your name etc on file then you have certainly have to apply thru them for a visa, but if you have not contacted them they dont know sweet all about you. I am presuming you have a Britsh passport ?
My pal went the day after her probation order ended, nobody bothered her and she certainly did not declare it, also they look for body language whilst you are in the queues after landing, try to act normal and dont look nervous, dont do anything that makes you stand out in a crowd.