In California, as with all the other states, voter registration is for sale to any party. Here is a list of fields in the voter registration file.
Given that info (which includes, name, date of birth, and address), it would be really simple to cross-check that info with commercial credit-info databases to see if a given person is registered to vote in multiple places, and whether he has.
It would also be easy to cross-check if a registered person has paid taxes, or is a legal citizen -- things the Dems would NOT want to happen.
Knowing Trump, he has almost certainly already gotten all this info via proxies, using his own money, a long time ago, and already has had research done showing where the most fraud has occurred. This is most likely a trap for state officials, where they can be charged with obstruction of justice.
The Post Office has a web feature, available to companies, to validate mailing addresses. Running the voter registration addresses through that, it would be easy to come up with a list of voter registrations to flag, including:
1) Registrations listing a business address (or vacant lot) rather than a residence.
2) Large numbers of people listed at an address which is supposedly a single-family residence.
Once in possession of the data in a standardized format, there are LOTS of computerized cross-checks which could be done.