There are ways to measure how accurate a system of probability prediction is across multiple samples. If the dem-rep predictions are 90-10, 80-20, 70-30, and 60-40, then that's a total of 300-100 across 4 elections, so there should've been three dem wins and one rep win during that period. If it was 4-0 or 2-2, those would both be equally wrong, despite the prediction always showing dems favored.
538's methodology is constantly being "refined," so the prediction system you'd be measuring would really be Nate Silver's ass. But you could still measure it.
There are ways to measure how accurate a system of probability prediction is across multiple samples. If the dem-rep predictions are 90-10, 80-20, 70-30, and 60-40, then that's a total of 300-100 across 4 elections, so there should've been three dem wins and one rep win during that period. If it was 4-0 or 2-2, those would both be equally wrong, despite the prediction always showing dems favored.
538's methodology is constantly being "refined," so the prediction system you'd be measuring would really be Nate Silver's ass. But you could still measure it.