You can taste it. Fast food ground meat has that signature old dairy cow flavor.
With chickens it's even worse: males are killed immediately after hatching, because their meat is worth next to nothing. Females are kept for both eggs and meat.
Poultry farmers raise both sexes and slaughter as soon as the bird is large enough.
You can also go to specific chains and look up their sourcing; Wendys sources from "steers and heifers", but all much younger than the average dairy cow cull age.
It's not that I have a high opinion of fast food beef. I just think it's unlikely that they could come close to meeting demand by sourcing from culled cows.