Partly it's pattern recognition, and ability to spot variances from your known patterns.
There is also, I think, some inbuilt fingering knowledge that you apply to sightreading. It's not actually about optimal fingering, more about "safe" fingering. Spot the top and bottom of runs, so you have a vague idea where you're going. I tend to cross fingers over more than is optimal as a precaution against running out of fingers (not saying this is a good thing, btw, just something I do), and an ability to do some strange finger crossings or swaps is also an advantage.
Knowledge of scale fingerings may be useful, but it's surprising how often a scale is embellished with extra notes or curiously missing a few (or both) in practice, and you need to be able to spot and deal with these.
And it is completely unrealistic to expect to read these runs at the same speed you can do them after weeks of practice. Your basic technique is a limiting factor here. It seems trite to say, but no amount of sightreading tips and tricks is ever going to allow you to sightread things you can't actually play.