Various journals claim to be indexed by worldwide databases like INSPEC, Google Scholar etc. How can I verify these claims?
Google Scholar is not a normal indexing service, in the sense that it does attempt to index only "trusted sources." You should be somewhat wary of any publication venue that advertises itself as being indexed by Google Scholar, since this is practically meaningless.
For INSPEC and many other indexing services, the service itself may have a list of indexed journals which you can find on their website. (See here for INSPEC's, for example.)
If your institution subscribes to the indexing service, you can check yourself if the journal is indexed there.