A false set is when you have the mushroom/spool resting halfway on the shear line, like so:

The plug rotates as if the pin has set on the shear line, yet the pin is actually still blocking the rotation of the plug. Hence, a "false set".

All you have to do is, when you set a pin, continue to push it upwards... if the plug starts to counter-rotate, it means you've found one of the mushroom pins. If it doesn't set properly when you push it further like this, take a mental note of which one it is so you remember not to ignore it later.