Every prerecorded cassette that incorporated Dolby noise reduction should have a Dolby logo/marking on the tape or the box. As I recall there were many prerecorded cassettes that were made in Dolby B, but I never saw any prerecorded tapes in Dolby C. I personally recorded many cassettes in Dolby C from material off of CDs. I had excellent results, especially when using high-bias blank tapes.
To address your question, you should not assume that prerecorded tapes are Dolby B unless they are marked as such. In any case, if the playback sounds best to you with the Dolby B switched on, then go ahead and listen that way you wont hurt the tapes.
To address your question, you should not assume that prerecorded tapes are Dolby B unless they are marked as such. In any case, if the playback sounds best to you with the Dolby B switched on, then go ahead and listen that way you wont hurt the tapes.