1)Try to hide in a house, with a door close while a zombie is chasing you. They will always go through the wall, door, etc objects. This is especially important if you are trying to hide, and finding out that the zombie is virtually unstoppable.
2) Take any object, blunt/sharp, and whack at any angle. You will notice they will stand stiff, bearing no damage, or not showing reaction to getting hit at any angle. They're body should flail or swing in the direction of hit (especially head and arms).
3) Try gathering a group of zombies and circle around them, or zig zag. They should be either be knocking each other over, or straight up shoving into each other.
4)Hit a zombie with any object, say an axe, and it will not splash in the correct direction/angle of impact.