When walking around (prob not many places in CBD) look for the "洗衣" characters..

Or just tell them to ask the hotel. Particularly if someone in their party speaks some Chinese, the staff at most of these places (all in my recollection) are more than understanding. Here's how it would go (or has for me).

Guest: Do you know where I can do my laundry?

Staff: You can do it here in the hotel

Guest (smiling): But its really expensive, I mean I can't really afford that 40RMB to wash one pair of underwear (insert simple clothing here). Would you do laundry here?

Staff (smiling, maybe complimenting on how "good" the guests Chinese is - though less and less at higher end places): Well you could go [insert address/location]. Or, sorry there is nothing here, everything has been built up.

Another possibility is to call up housekeeping and just ask. It can't hurt.