The pool validates each and every share submitted by the miners. Each of the shares have a certain difficulty and thus a certain weightage pegged to it; an ASIC submitting a share with a larger difficulty (ie. 50) would earn the same amount as one that submits 50 shares with 1 difficulty. The work done for both are proportional and thus the compensation would be proportional to the shares and the difficulty of it being submitted as well. PoW makes it such that it is expensive to find a share of sufficient difficulty but quite trivial for a hardware to actually validate them. It isn't a problem for pools to validate each share that the users submit, by the fact that pools often implemented PPLNS or that of sorts, it discourages miners from intentionally holding shares that meets the actual target as this lowers their remunerations.

Tl;dr: 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.