generally speaking for han chinese folks, surnames are one character and given names are one or two characters (with two being more common, usually). so if you see a name that's 1-2 or 2-1 (i.e. liu yifei) and it's not one of the few known multi-character surnames, then you can safely assume that's their given name.
for a 1-1 name like yao ming, it's a little more difficult. some characters are definitely more common as surnames than others - the chinese term for 'common people' (百姓) actually refers to an old classic text where they compiled all the surnames they knew of! so when i see the name yao ming, i immediately recognize that 'yao' is a fairly common surname and 'ming' is not, and thus it's more likely (but not guaranteed!) that 'yao' is the surname here.
there's also some cases that are ambiguous when romanized, but not ambiguous in chinese. for example, consider the name 'wang chen,' where both 'wang' and 'chen' are common chinese surnames. however, if i saw it written out as 王, i would be able to recognize that 王 'wang' is a character that's primarily used for surnames, while 晨 'chen' is not.
for a 1-1 name like yao ming, it's a little more difficult. some characters are definitely more common as surnames than others - the chinese term for 'common people' (百姓) actually refers to an old classic text where they compiled all the surnames they knew of! so when i see the name yao ming, i immediately recognize that 'yao' is a fairly common surname and 'ming' is not, and thus it's more likely (but not guaranteed!) that 'yao' is the surname here.
there's also some cases that are ambiguous when romanized, but not ambiguous in chinese. for example, consider the name 'wang chen,' where both 'wang' and 'chen' are common chinese surnames. however, if i saw it written out as 王, i would be able to recognize that 王 'wang' is a character that's primarily used for surnames, while 晨 'chen' is not.