I'm having this exact same issue. I bought 5 chickens, the previous owner told me they were all 9 month old hens. When I went to pick them up one is DEFINITELY a rooster (he's much larger, huge wattle/comb, long curved tail feathers) but the previous owner had never seen him crow and apparently didn't know what a rooster looks like.
I suspect they might be younger than he thought, because there's also no eggs to speak of either ... but my fingers are crossed he never learns to crow There is a hen who regularly puts him in his place too, so maybe he keeps quiet since he's not in charge?