There is usually no shortage of girls right in a hotel. Unless things have changed (and depending on the hotel you would stay in), you should find girls wandering in the hotel lobby, cafeteria and outside the karaoke bar or nightclub, if your hotel has one. The last time I was in Beijing a couple of years ago, there were tens of girls in the hotel lobby alone and many more at the bar.
It is easy to tell the girls in the business as opposed to hotel guests. Just make some eye contacts with the girl of your choice. If she were in the business, she would either approach you directly or give you a smile back.
If your hotel uses room keys that have room # on the key, one trick I’ve learned from the locals in Beijing was to flash your hotel key in front of the girls. This is a signal that you are in heat, LOL, and before you know, you would be overwhelmed with phone calls and even girls showing up at your room door!! I tried that once when I was in Shanghai, and men, I was busy answering calls and the door for the next hour.
I’ve never been to Guongzhou but I understand from my friends that there are lots of nightclubs where the girls would offer outcalls. For foreigners, the advice is to go to the bigger clubs only, and avoid dingy little joints, as they could be risky. The bigger clubs might cost a bit more but they're a whole lot safer – bear in mind that “sex-for-money” and Johns in China are illegal by law.
I don’t know the current rates or the rates in Southern China, but when I was in the northern cities, RMB 800 (around Cdn$130) bought me an overnight service, unlimited shots.
Hope this helps, and happy journey.
