put the helmet on your head, and do up the strap. Hold the mouth guard area lightly and attempt to turn your head inside the helmet. The cheek pads should immediately stop any rotation and the mouth guard should be being pulled out of your had BEFORE any significant rotation of your head.
Fit should be snug, and comfortable without any one area being pressured by the shell of the helmet.
Try a few different brands, I use a Troy Lee despite being able to get Kali at trade. The Kali helmets are awesome and so so light. Tony and a load of the others I ride with use ’em. I’ve just got a massive noggin so run the Troy Lee in Carbon. Don’t buy the cheapest you can, try loads on, pick the one you want.
PS doing up the strap ensures that the sides are properly supported when trying it out for fit. Leaving them undone allows the helmet ‘sides’ to flex