A fuse is blown if the metal link inside the fuse looks like it has separated. With older fuses that are made of glass, it is possible to identify a blown fuse by looking inside the glass and checking to see if the link is intact.
In some cases, it isn't possible to identify a blown fuse just by looking at it, because a bad fuse may appear intact. The most reliable way to test a fuse is to use a voltmeter while the ignition is on. It is important to ground the voltmeter lead before doing this. If the meter reads current on both sides of the fuse, the fuse is good. If the meter reads current on one side of the fuse and not the other, the fuse is bad.