So are audiophile fuses really directional? My experiences have been limited and I found them to be marginal improvements compared to cables, room treatments and speaker positioning. I did find that the Swiss Digital Fuse Box made a greater impact sound wise than fuses in my systems. Yet I am not saying they cannot make a difference. YMMV.
Some are directional by production method, apparently having to do with the direction the wire on the spool is pulled. I assume those developers indicate direction on the fuse. In other cases, it's either trial and error or is created from the way the fuse is burned in and used. With the wrong direction the sound will be spatially pinched, vs. the other direction. In principle, the best direction is easy enough to check. However, it ultimately depends on type of fuse holder and how easy the fuse is to get to. For some, it's just pop open the fuse holder in place with a small-bladed screw driver (power disconnected); for others, I've had to pull the component out of the cabinet and, in one case, flip it over. While I've only had one bad experience, with audiophile fuses it's best to be extra cautious, and not just because of the money (one of the better AM beeswax fuses fell apart on me taking it out of the holder to change directions; afterwards, the guy who sold it to me claimed a $40 fuse removing tool was necessary, but that's bs -- it was the fuse construction not the method).