Most organizations require a specific Snell (SA) safety rating. There should be a sticker/label in the interior of the helmet that shows the helmet’s SA rating. You might have to move some of the interior padding around to find it.
These ratings are updated every 5 years and most organizations allow the ratings to be legal for 10 years. So for example an SA2005 helmet would be legal until the end of this year, and after that it would be illegal.
New helmets are currently on the SA2010 rating. The SA2015 rated helmets should be out later this year.