Typically, if there’s swelling on the outside of the skull, the brain underneath was protected.
But the most useful tidbit I can pass along is that if your kid develops a massive goose egg within minutes, then in all likelihood there’s no concussion.
The best advice I can give you is: when in doubt, call your doctor. Plus, babies and toddlers have such large heads relative to their bodies that when they fall, they almost always lead with the head. Parents of babies and toddlers can be especially freaked out because their kids cannot articulate their symptoms. Want something written for a non-scientist? There’s good information here and here. If you are into reading science-y papers, check out this one, perfectly titled Concussion is Confusing Us All. Parental worry around concussion is completely legit. They can include a long laundry list of other symptoms, though, including headache, double or blurry vision, ringing in the ears, confusion, personality changes, nausea, vomiting, sleepiness, I could keep going. Today, 90% of cases do not involve loss of consciousness. This may not have been the medical definition back then, but it certainly was a general consensus. Back in the 70s and 80s, concussion implied hitting your head so hard you passed out. Some of this is a function of the fact that we have redefined the word concussion. I remember one, maybe two kids having concussions when I was growing up.