When you have the helmet on and the strap tight, reach back and push up on the back of the helmet, it should roll forward a little, but not enough to block your vision. Then push up on the chin, again it will roll a little, but not much. Next grip the helmet with a hand on each side, and try to twist the helmet right and left, without moving your head, preferably infront of a mirror. Again it will move, but it shouldn’t move much, and when it move your cheeks should get pulled along with it. If a helmet passes those tests, see if the store minds you wandering around the store with it on for about 15-mins, sometimes a helmet will feel right at first, but after a while you get a headache, which is bad when you are on two-wheels.
The instructor in my basic course suggested that if you are buying a ‘better’ brand helmet, they usually have removable/replaceable cheek-pads, so you should first try them on with the cheek pads out to pay more attention to the fit around the crown of your head, then when you have that fitted find the right cheek pads to go with it. Also the SHAPE of a helmet varies depending on manufacturer and style of the helmet, so depending on the shape of your head some helmets will fit better than others.
If you want something visual, there are some good videos on youtube, here are a couple that I have found.
Proper Fit by The Helmet Center
Helmet Fitting by JPCycles
If you look at The Helmect Center’s channel, they have reviews of different helmets too.