Fuses come in all shapes and sizes, but two of those shapes and sizes are far more common in ordinary consumer electronics than others. While not necessarily standard, they’re common enough to comprise the bulk of all cylindrical cartridge fuses you’re likely to deal with. If your device has a replaceable cartridge fuse, the odds are very good you need a fuse in one of two sizes - 5x20mm or 3AG.
These two fuse types look largely identical, visually - both are glass or ceramic cylinders with metal caps. To tell the difference without tools, simply compare the length of the fuse to the diameter of a typical U.S. quarter. A 5x20mm fuse (being 20mm in length) is slightly shorter than a quarter, while a 3AG fuse (being just under 32mm in length) will be longer than a quarter.
The markings on your fuse are unlikely to be a part number, but instead specifications. Look for a number followed by ‘A’, such as 6A, 10A, or .5A. This is the amperage rating of the fuse. There should also be a ‘V’ number, often values like 24V, 32V, or higher like 250V. This is the voltage rating of the fuse. With these three values - the physical size, the amperage, and the voltage - you should be able to narrow your options down to fuses that will work for you. The other significant factor to keep in mind, if able, is whether the device your fuse is installed in has a motor. If yes, you should probably acquire a slow-blow fuse. If not, a typical fast-blow is your best bet.
Remember, if you need help, you can always contact DigiKey Applications Engineering.