You might indicate time leaping by having the character experience d茅j脿 vu or strange memories that don't fit the current time. Or maybe they have a special device or power that causes the leap and it's mentioned explicitly.
A simple way could be having the narrator directly state that a time leap has occurred. Another option is to show the character waking up in a different time period with no explanation at first, leaving the reader to figure it out as the story progresses.