In major China cities such as Guangzhou and Hangzhou, taxis are equipped with fare meters and machines to print receipts. Ask for a print receipt when you leave the taxi, which usually shows the taxi fare, the taxi number and the license number of the taxi driver. This receipt would be useful if you suspect that you are ripped off and want to lodge a complaint.
Nowadays, because of the rise in fuel price, many cities allow for taxi fuel sub-charge ranging from RMB0.5-1.0 per journey. Say Guangzhou, which imposes on passengers a fuel sub-charge of RMB1 per taxi journey. So do not be surprised if you are asked to pay RMB1 more on top of the charge shown by the taxi meter.