Pools do check for the validity of the shares and the shares represents their contribution to the pool; lesser effort = lesser rewards and zero effort = no rewards.
If a share doesn't include a single PoW that solves the block however, but only attempts to solve the block, what is the precise mechanism for proving the PoW value is not just an unchecked random number? I have an idea, but I'd prefer to hear someone who is in the know explain it.