Default Units of length used in datasheet
There is no standard but it's not usually difficult to tell.  In this case, the overall dimensions of the relay are given as 10 X 20 mm right near the top left of the first data sheet page.  If you look at the dimensional drawing, the overall dimensions are also given as 10 x 20.  Therefore, it is clear that the units of the dimensional drawings are mm. Since there are 25.4 mm to an inch, it should be obvious in most cases, whether the units are mm or inches.