After you have made your visual check, grab the helmet in your hands, one on either side, and try to rotate the helmet from side to side. Note any movement of the skin while doing this, as well as the amount of resistance to movement remembering to hold your head steady. Next, check movement up and down, again noting skin movement and resistance. A properly fitted helmet will cause the skin to move as the helmet moves. And, it will feel to the wearer as if evenly distributed pressure is being continuously exerted around the head.
Helmets are a little like shoes, in that they do break-in a little.
For this reason the best attitude to have when fitting is that the helmet should be as tight as you can stand to wear it. Now, fasten the chin strap, so you can check it. After the strap has been tightly fastened, while holding your head steady, reach over the top of the helmet grabbing the bottom edge with your fingers. Then, try to roll the helmet off your head. If it comes off, it is undoubtedly too large.