How can I tell if my 2 month baby has brain damage from a recent fall and concussion with subdural bleeding. He acts normal, eats well, cooing etc?

Would have symptoms. If your infant has a concussion, symptoms would include altered alertness and responsiveness, a bulging fontanel, decreased eating, vomiting and excessive sleeping.
Time. Difficult though this is we are going to have to wait and see. It sounds as though everything is going well, but we will just have to watch and hope. Good luck.
Follow up. Babies have a lot of plasticity of brain. Usually if they seem normal after the treatment for the hemorrhage, the prognosis is better. But the baby needs follow up with a pediatric neurologist at some point of time in the next couple of months to review baby for any changes.