Ok, so the verification process has two aspects.

The verification of academic achievements is done directly by the universities and employers; they are incentivized to do so as this allows them to both protect their reputation as-well-as earn SPRING. Have a look at my previous post, for additional details. :)

The legitimacy of endorsements is guaranteed by the fact that endorsements aren't free. So, people won't just hand these out as there is a cost attached to them. The endorsements return a value only if the person you endorsed gets hired or paid; you receive a portion of the finder's fee, which is shared by all those who endorse the person. So, people will have to carefully take into account whether tor not heir endorsement will yield an ROI.

Also, spam endorsements won't be possible. Others have the right to reject your endorsements. Racking up rejections will be costly as they increase your cost of making future endorsements.