To go over ground I've trodden before:
When you're shopping for a helmet you can go to the Snell Foundation website and see listings of each manufacturer's helmets that are tested and rated by Snell, right down to the model and (I think) sizes. Make sure the one you're thinking of buying is on that list.
When you purchase the helmet - be sure to specify that it must be Snell rated 20XX (fill in with your choice of year). I once bought helmets from one of the online places. I waited until the model I wanted was now listed in the Snell listing for the newest cert -- but didn't specify on the order. Sure enough, I got one that was five years old (by rating, at least). When I asked/complained, I was told they were selling the old stock first before moving on to the newer-rated models. If I had specified on the order I would have had recourse -- as it was -- I was SOL.
And when you get the helmet that you've specified must be of such-and-such Snell vintage -- dig deep 'til you find the Snell cert. sticker. They are NOT what you might see on the outside of the helmet, but rather are on the inside of the helmet, well-hidden (usually) under the padded lining. You'll have to dig to find it, but whether you want to dig that deep or not -- your helmet inspector in the tech line WILL dig for it. You woudln't want a nasty surprise, would you? By the further way, the Snell cert. sticker is a rotogravure-process printed label -- that is, on shiny metallic foil and usually pretty difficult to read in the dark confines of the helmet. But if the helmet is Snell rated -- that sticker will be there and will give you the date.