I was riding with a friend who is a much stronger rider than I, and who also has far more training in emergency medicine than I. When I caught up to him after he went over the bars, he was sitting in a pile of rocks with the wheels of his overturned bike spinning in the air behind him, and he looked up at me with his finger pointed ninety degrees in the wrong direction and said, "I don't know what to do." It was totally unfair. I'm just an idiot. You can't take me along on a ride and expect me to respond to something like that in a meaningful way.
Once he set his dislocated finger, which he later found out was broken, we checked his helmet for signs of impact. There were none. We sat under a tree for awhile and he seemed completely himself. There were no signs that he'd suffered a concussion, but a few hours later he was slurring his speech so badly that he was unintelligible.
So how do you diagnose something that's extremely dangerous but invisible? Some of the signs are obvious. Dizziness, delays in communication, emotional instability, poor coordination or balance, slurred speech, problems with memory or concentration, nausea or vomiting, and confusion. (Obviously, if a friend takes a spill and loses consciousness for any amount of time, the ride is over. Even if they insist on continuing, you need to walk their bike back to the car for them and drive them to the doctor.)
This is how your friend may behave if he or she has a concussion:
If your friend takes a rough fall, take a long rest. Wait twenty or thirty minutes before resuming the ride. Ask lots of questions to assess their mental acuity as well as any symptoms they may be feeling, as they may not recognize that they are feeling them until you ask. If they are pretty shaken up, they may have a concussion even if they didn't strike their head. If their teeth or ears are ringing, their jaw may have clamped shut fast enough to give them a concussion.
Nobody likes to cut a ride short, but continuing to ride with a concussion is about the worst thing a person can do. The health of your friend's brain is more important than recreation. If you in any way suspect your riding partner has a concussion, take it very easy, get back to the car, and make sure your friend won't be alone for the rest of the day in case symptoms develop. Don't let them engage in physical or mental activity. Stay out of the sun and avoid alcohol.
It's a tough line to draw. You shouldn't call of a ride just because your friend has a scratched leg and says she's okay. She probably is. However, if she takes a really rough fall, take your time. Be honest with each other. If there's any doubt, call it. Don't leave your friend alone in case she becomes confused. Use your brain, because she might be unable to.
Anyway, this ends the safety lecture from your mom. Take care of each other.