Usually upstream ISP providing transit accepts only a valid set of prefixes that they have agreed to advertise on the public internet from an ISP customer, they enforce a policy on the ingress to make this happen. Idea being, if the customer ISP ends up advertising an incorrect prefix, then the impact is only localised to his ISP and not to the whole world. But some ISPs don't follow this and implicitly trust the customer ISPs and of course there is no cover if the tier1 ISP itself typo's a prefix. There are tools such as BGP RPKI available, but its not widely deployed.