Keep in mind that slow Blow and fast Blow fuses are not interchangeable. If you put a fast blow in a circuit that was designed for a slow blow it will likely blow out on some startup inrush currents. And if you go the other way around you risk damaging the equipment if the fuse does not react fast enough.
The first place I would look is on the equipment itself. The rating of the fuse is usually printed on the case right by the fuse cap and if it requires a Slow Blow it will say that. If there is not indication on the case next place I would look would be the owners manual. (If you don't have one manuals for damn near everything are available online.) If you bought the unit new you can look at the fuse that is in it they usually have the ratings engraved in the end caps. But if you bought the unit second hand there is a chance that whomever owned it before you changed the fuse and you have now way of knowing that it was replace with the proper one, so that can be risky (who knows if they blew a fuse and all they had was one with a higher rating than required and put it in on a "temporary" basis, only to forget to replace it with the proper size... wouldn't be the first time.
FYI - Fast Blow uses most commonly look like a thin piece of wire going through the case, like this:
Whereas Slow blow fuses usually have a coiled wire and sometimes some other stuff inside the cartridge like this:
or
Of course this assumes you are looking at a fuse with a glass sleeve. If you have one with a ceramic sleeve you won't be able to see a thing.
The bottom line here it that it is very important to use a fuse that matches the electrical rating that was specified in the orioginal design and it is well worth the time to research what that spec is.
I hope this helps.