This guidance is based on my own experiences last fall:
1. Only expect a specific RS discount if it is actually mentioned in the description of the hotel or B & B in a current RS travel book; of course a hotel may also offer other discounts for early booking, cash payment, several nights, prepayment without right to cancel, and they may or may not allow use of various discounts stacked on top of each other.
2. Check out the hotel's reviews, availability and rates on sites like booking.com, tripadviser, etc.
3. Then go to the hotel's own web site and check what it says about availability and rates, which might be the same or different than what the booking sites say. The hotel will usually list any generally-available discount for prepayment, early booking, etc. but probably will not mention any RS discount. Sometimes you need to contact the hotel or B & B to ask about availability and rates, using their web form or e-mail, so do that.
4. At this point, I would send an e-mail saying something like: "I will be visiting your city for x nights, on these month and dates, and your web site (or booking.com, etc.) lists a rate of xxx euros per night. I will be traveling with my Rick Steves travel book, which mentions a special discount of xxxx at your hotel. Does that mean my rate would be xxxxxx, and how should I reserve a room at that rate?" Personally, I always add a couple friendly comments to establish something of a relationship, which is almost always clearly appreciated.
5. Then wait for a response and do what they say. Most are used to questions like this, know you want their best rate, and have someone who can respond in English without any problem. Print out and take with you some e-mail record of the message confirming your reservation and rate.
I did that at 2 or 3 different hotels for last October, and it worked fine. All were positive experiences. I don't think that any of them actually asked to see my RS book.
Good luck!