What is the meaning of 确定 in 你确定你是外国人?
確定 as a verb means "to determine" 

「確定你是外國人」literally means "determined you are a foreigner" 

To illustrate the meaning of 「確定」 as a verb more clearly, let's look at the following sentences:  

「可以確定這是假貨」( can determine this is a fake)

「可以肯定這是假貨」( can be certain that this is a fake)

「名單已被確定」( the list has been determined)

「名單已被確認」(the list has been confirmed) 

"Are you certain you are a foreigner?" -「你肯定你是外國人?」