In some rare cases, a fuse is in a circuit that is at a 'float' voltage, then goes to ground when active.
The 'float' voltage is unknown, as it represents the state when nothing is connected.
Usually, a fuse passes 12 volts in a car, so both little test points on top of the fuse should measure 12v, or whatever the current battery voltage is.
If you read 12v on one side of a fuse, and not on the other, then that is a blown fuse.
You can see the two test points in this picture :