Don't depend on a door chain when checking to see who's at the door; some aren't strong enough to prevent a forced entry. The safest way to find out who's outside is with a peephole, or door viewer they can be installed in wood doors and UPVC.
Although you can simply install a swinging cover over a hole in the door, you're better off with a wide-angle peephole that gives you a 180° view of the exterior. Make sure that you install it at a convenient height for the shortest adult in the family.
If you don't want to make a hole in your front door, you can mount a mirror above or beside the door (or on a nearby window frame) at an angle that reflects a visitor's image and in a position where it cannot be tampered with or obscured.