Identifying reputable journal indexing services I am part of a committee tasked to create a policy on creating incentives for publications of university employees.  Right now, the highest incentive award is given to publications in journals "indexed in Web of Science or Scopus."  I would like to rewrite the policy so that it is more general, that is, open to the possibility that there are other highly regarded bibliographic database and index services.  I am aware that there are many predatory journal indexing services, and I want to exclude these.

How do I define a reputable journal indexing service?
I think that there is a critical distinction to be made here between defining reputable indexing services and identifying such services.

I suspect that for most well-established scholars, it is relatively straightforward to identify whether an indexing service is reputable or not. For example, if you talk to a computer scientist, they'll point to DBLP, MathSciNet is a good one for mathematicians as noted in the comments, and PubMed is good for life sciences.  Random predatory anything is usually pretty easy to determine with a little bit of research.

In order to define "reputable indexing service" for the sake of policy, then, I would suggest that it is likely to be effective to have a process by which faculty members can request an index be considered and then a few internal and external experts in the field are asked to certify whether the index is reputable or not.  That way, there's not a formal definition enshrined in policy that might be evaded by technicalities, but instead some human judgement is involved.