How can I tell if late eggs/larvae are part of the reproductive or migratory generation?

In short, the fall migratory generation isn't cleanly divided from the later summer breeding population. If a butterfly is reproductive, it won't successfully migrate and overwinter because reproduction requires a huge investment of their body/lipid reserves, so they don't live as long once reproductive. Reproductive diapause is when monarch's reproductive organs are delayed in maturing - they wait until the next spring at their overwintering grounds. Once their reproduction organs mature, they'll continue to lay eggs on milkweed until they die. This doesn't bode well for a window of late-season eggs/larvae with limited success in fully developing and completing the migration. That said, the 'pre-migration' phenomenon does show that reproductive monarchs are moving south and continuing to lay eggs. This doesn't mean they'll make it to Mexico and survive the winter. Because the last of the reproductive butterflies from July/August continue to lay eggs until their death, and the migratory butterflies are emerging at the same time, there is no exact time that all butterflies you see are either one or the other (reproductive or migratory).