These days, we’re lucky – we have the internet!
The best bet is to google their name. Odds are good that someone on the ThornTree website or elsewhere (a travel blog) will have mentioned their name if they’re big and reliable, and also if they scammed them.
Try searching their name plus words like “problems”, “successful”, and the like.
Alternatively, you could ask another question on here asking for a recommendation for an agent to help with this. For example if you were asking about Russia or the ‘stans, I could recommend a few, and I’m sure others would have experience with visas for China.
Credit:stackoverflow.com‘