I've seen bent chain rings before that had to be removed and hammered flat. Generally pretty easy to push the chain off, spin the cranks, watch the chain wheel from the front, and see if the teeth seem to gyrate side to side as it rotates. Happens when people bash it against an obstacle, I suppose.

I've also seen worn teeth on chain rings/worn chains where the chain no longer sits flat on the chain ring, but leaves a gap under it. If the chain ring's teeth look non-symmetric or sharp and worn down, that's usually a tell the chain ring has to be replaced.

For chain stretch there's cheap checker tools you just drop down on the chain and how far it drops tells you how worn out the chain is.

Could always be the crank coming lose too, though, like people have already said. The chain wheel could move outward giving poor chain line.