How do you know?

Yesterday I ran the 12Ks of Christmas in Kirkland. The 12Ks of Christmas is a play on the 12 Days of Christmas, which each K representing one of the days...three french hens, etc. You'd think that would be pretty obvious, but you'd be surprised at how many people were confused by this concept.
Anyway, I digress. It was a fun race and the weather was beautiful...clear and brisk. I felt good and the race went well. I'm not sure what, if anything, was off and yet I still believe that I am capable of running faster than I did. Was it because I was just training through it? Was it all those bloody hills? Just what IS the trick?  How do you know how fast you can truly go? What are your true limits? I wish that I could have one perfect day when, at the end, I would know that was it. That was as fast as I can go and that is the gold standard. But there is no way to know is there? You will always wonder...even if you meet your goal, set a new PR or feel like a super star all day.
I have also decide that someone needs to create a magical formula in which you can plug in your hours of sleep, the week's workouts, nutrition, weather, mood, course, your bathroom habits, etc. and out would come a predicted race time. Surely some super computer could work on that.