First: You don't need a sign to know wheter a hyip is about to disappear or not. At the first day the hyip has popped up, there is already an assurance that it will be gone.

Now to know if a hyip will last long, take note that you should always sew if the admin/owner of this hyip are responsive. Watch their words. You can see by it the time they are going to last long. Also be sure that a hyip is popular enough to have all the investors needed to pay all that is in cue. A hyip without new investors is already dead.