Op’s question: How to determine if you need more fatigue resistance (or better, I guess). Not how to accurately measure fatigue resistance.
My ways, have an accurate understanding of where some efforts should lie in terms of power and feel. Something like sweetspot or underovers is good for me. Then do these after 2500kj/3000kj and good fueling and see how the rpe or power stacks up. If around 5% on non maximal efforts then I think it needs work. I can do under overs fine and consistently after 2500kj or 3000kj, but start with 1500 or 2000kj at first.
Another way is just through racing. If you find in races you struggle to put out good power compared to others of similar ability, then obviously something needs to change. A perfect “test” would be a race that finishes on a ~20min climb. So you can gauge your power data after 2500kj to your fresh numbers. For something not training massive hours a bit over 5% would be acceptable.
EDIT Just thought about this 20mins after. Biggest thing to think, is it a limiter? Are you limited in your performance because of your fatigue resistance or because of purely your strength including when fresh? Easy to think it’s fatigue resistance when you get dropped after 3 hours because that’s when the race was hardest, but if you would struggle to do that pace near the start of the race (or if you were just declined the whole way) then you probably just need to get stronger. If you’re one of the strongest in shorter races but start to faulter when they get longer, you either need to eat more or work on your fatigue resistance (likely both anyway).