You are correct in thinking it's not possible to tell the difference. For example:
이번 설날에 선물을 보낼 친구가 하나도 없다. I have no friend to whom I will send a gift
in this Lunar New Year.
선물을 보낼 친구가 전화해서 주소를 물어봤다. A friend who would send me a gift called me
to ask for my address.
No. 2 is clear because there will be little or no chance that 'a friend to whom I will send a gift' would call me to ask my address. The phrase will not be confusing if it is supported by clear context.
If you are worried about the chances the second sentence might sound a little off or awkward, you could consider using the following sentences:
선물을 보내려는 (보내려고 하는) 친구가 전화해서 주소를 물어봤다.
선물을 보낸다는 (보낸다고 하는) 친구가 전화해서 주소를 물어봤다.
It proves again how complicated the language is.