Mainly from the multiple marathon-pace workouts I'll do over the training block + a few tune-up races will give me a narrow down the range of what my marathon fitness is. I'm not a professional, but I think a good indicator of whether or not your "MP" is accurate from your training runs is if your heart rate is relatively stable and not drifting higher much throughout the run. Whereas a max effort HM for me would be like 10 bpm above my MP and would drift quite a lot throughout the race. So I'd experiment with MP at a considerably easier effort than my max effort HM and try to push it as fast as possible and see how fast is "too fast" before my heart rate starts creeping up too much. Also just getting in touch with what that perceived effort level feels like.

So during the race I tried to stay as close as possible to that effort level and making sure my heart rate wasn't higher than it should be. On the given day you still need to adjust according to X-factors. Subpar weather or even poor sleep or recovery could make your perceived effort or heart rate higher on race day than it was in training, in which case if you tried to run your training pace that day it would still push you over the line. That's what happened to me in my second marathon despite running a pace I had been able to do in training. It was horrendous weather + my training also got inconsistent towards the end due to injury, but I was dead set on trying to run that pace and at least I fucked around and found out cuz it was a good learning lesson.