Adding a pilot light to a light switch
Depending on your setup you may find it more convenient to do as Ed Beal suggests but instead of installing a pilot light put in a 3-way switch with pilot light. This way when you find out the basement light is on you don't have to walk downstairs to turn it off.

Remember that you will need enough conductors in the cable for the hot, neutral, travelers, and ground. And that you will have to swap out the switch downstairs for a three-way (no pilot).