in addition to an up to date SNELL rating you also want to look to see if the helmet is a "SA" or "M" rating.
SA helmets are rated for multiple angle impacts, such can happen in a roll over with a roll cage or side impace where your head can physically come in contact with multiple objects from multiple angles.
M helmets are simply a motorvehicle/motorcycle rating. It is rated for a single point impact. Such as hitting the ground after your bike goes down.
If you are only planning on using the helmet for motorcycle use then M is fine, however if you plan on using this in your car on an open track, autoX, etc. then definately spend a little extra and get an SA rated helmet.
