They can identify that the people controlling the mob are not from Hong Kong by their accents. “Putonghua” is a reference to Mandarin, spoken on the mainland. Hong Kong folk generally would speak Cantonese or some local dialect, not Putonghua. China is afraid to attack the protesters outright, so they send in these goons to create chaos and break the protests up.
Via Time:
Attack comes on the sixth day of democracy protests, with many students exhausted
Protesters take a break as the block the entrance of Hong Kong’s Chief Executive Leung Chun-ying offices in Hong Kong
A pro-government mob, hundreds strong, destroyed one of Hong Kong’s democracy-protest sites Friday afternoon local time, attacking students, trashing student tents and hurling obscenities.
Police were initially overwhelmed and failed to separate the two sides, although they were able to evacuate some democracy activists from the site, located in the densely populated district of Mong Kok, on the Kowloon peninsula.
The anti-Occupy crowd destroyed banners and posters and chanted “go home” at the students. Water bottles were thrown. Abandoned buses that had stood at the site since activists occupied it were boarded and driven off to loud cheers.