Just wondering how you can tell if you have given your baby/toddler enough food and how long should it really sustain them?

EG. last night i gave my son dinner at 6.30pm. He had alot of meat and veg as well as fruit afterwards. I could tell he was getting slower with each mouthful and just had that 'full' look about him like i know i feel when eating alot.
He woke up at 9.30pm and had a huge bf. He would just cry and cry until offered him milk. I know babies can digest breastmilk in 2-3hours, but solids? Does it digest as quickly as well?

If it was just a comfort thing then how do i tell the difference?