How can we know, today, that there's something from 100 light-years from here?

Because it's not just a picture of something 5.87862537 x 10^14 miles away (the total distance in 100 light years), it's 100 years in the past. In other words, it's already happened, and it's simply taken the light (or other electro-magnetic radiation) that long to get here. If some aliens were pointing a camera at us from 100 light years away, they could observe all manner of interesting events.

For instance, the Earth is actually about 8 light-minutes from the sun - it takes 8 minutes for it to get 'here'. If the sun was to go out suddenly (a terrible tragedy, to be sure), we wouldn't find out... for 8 minutes (err, just eyeballin' it, no special detectors).