You can find the information on weather web pages, but in my opinion it is normally not too rainy. My experience: there are quite often a few very nice days (almost sommer ) just before the end of october.
I have been in Prague three times now in October - a little cool sometimes, overcast sometimes, a bit rainy sometimes, but, most of the time, just wonderful days (I am planning on going back in September/October again this year)!
Hi artifex, check out our Fall/Autumn Weather in the Czech Republic page where we talk about October weather. Dana