If you hover over any unit, you can judge the food level of the entire group by looking at the yellow food percentage bar on the tooltip. Units automatically share food between all other adjacent units so they have comparable percentages of food e.g. a unit of Hoplites with no food approaches Hoplites with 100% food, and they'll automatically divide the food evenly so that both units are at 50%. Phalangites with 180/180 food approaching Peltasts with 0/40 food will share the food so that the Phalangites have 147/180 and the Peltasts have 33/30, or roughly 81% each.
This means that large groups of units will each individually have the same percentage of food as long as they're close to eachother.
To take advantage of this, one strategy I've used in prolonged (starve-out) sieges to ensure my sieging units don't starve while in dangerous territory is having a group of 2 or 3 tribal cavalry approach nearly starving friendly units. They'll equalize their food supplies, and the cavalry can go back to a city to pick up more food without fear of being routed by stray peltasts or cavalry like workers or slaves would.