But how does someone find out his life is cut in half? like if that person is supposed to reach the age of 80, dies at 40, how would people guess its because of the devil fruit? wouldn't it be more normal to assume he just died instead of saying its because of the fruit?
Other people have mentioned the possibility of some kind of index fruit or that the user might feel their life getting shorter (how scary would that be?) You could probably figure it out by just looking at the deaths of the past users.
Devil fruits seem to have been around for a long time. Let's say they've been around for 800 years since the Void Century, at least. If the user repearedly dies at a young age over seven or eight centuries, you're going to start noticing. If the user is a dwarf or giant that difference is going to be even more apparent. Furthermore, it seems likely that the user dies either of mysterious natural causes or by problems usually only seen in the elderly, like organ failure. In fact, there's a good chance that the user could die immediately after a healing, which would make the connection pretty apparent.
Factor in the probable fame a renowned miracle worker would receive and it's easy to see how someone could have put the pieces together.