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Well, there are a couple things you could do:
-If you can't make out any rating info on the old switch, you could see on the fan motor itself if you can see any info on it.
-Or Bypass the switch, splice the wires together in the box, and read the current with an amp meter.
-If you can find a 10 or 15 amp rated switch, it will most likely be more than enough.
-There won't be a difference if it doesn't have 5 speeds, but the more control you have, the better.