Most car seats' ISOfix connectors have indicators which are red when the seat is not in the car, or more importantly, when it's not connected properly.

When the ISOfix arms lock into place the indicators turn green.

Most support legs have indicators that turn green when the leg is firmly in contact with the floor.

The indicators on this base are red when the base is not connected to the ISOfix points...

... and as soon as you click the base into place, the indicators turn green.

These indicators show that the seat has been correctly attached to its base.

Top tether straps have indicators that turn from red to green when they have been pulled tight enough.

Some car seats have extra side impact protection, either as a detachable block...

... or built into the seat. It only needs to be attached or opened on the side nearest to the door.