Still being green means nothing about ripeness.
It isn't a watermelon though - Watermelons have oak-like leaves and tiny flowers that are pale in color with large, very prominent genitalia.
Normal melons have rounded leaves, but not quite as lobed as yours, with small bright yellow petals and the genitalia is hardly there.
If it is something that grew from what you paid for, it will not be the same if you saved the seeds, it will vary. Almost all melons from the store are hybrids, and thus, will not breed true even after the second generation. Yours may vary slightly or by a huge amount.
Still, I'd cut one open, ripe or not, and show the inside. I can help you identify it a LOT just by seeing the flesh color and rind color, seed amount, etc. Also, do any of them smell at all? Tap on them, how do they sound? Do the tips smell much?