I fell holding my friends 8 month old baby. we went down pretty hard. i got rug burn on my knee. we fell onto tile. he started crying right away.

Fall: You did not say if the baby was hurt at all. Crying after an episode like this may be just from the sudden movement. If the baby is acting normal has no evidence of injury , did not hit its head, is moving all it's extremities, then the chances of anything significant is small. If this is not the case or if you have concerns take the baby to the ER to be evaluated

I assume: your question is on how to tell if the baby is OK. As long as he cried immediately (he might have gotten a little sleepy right afterward), is not vomiting, is acting relatively normally, and is moving all parts of his body, he's probably OK. If he starts doing any of the above, or anything else that seems concerning, take him to the ER. Hope this helps!