I was born in Quincy,a town on the east coast of America 后面是

很明显是同位语.最简单的方法如果是非限定性定语从句,那你试着想想把后面从句补齐,那应该是which is a town on the east coast of America之类的说法 ,但是很明显前面的which is 是不可以省略的,所以它是同位语,以后你也可以这样区分下.